(navigation image)
Home American Libraries | Canadian Libraries | Universal Library | Community Texts | Project Gutenberg | Children's Library | Biodiversity Heritage Library | Additional Collections
Search: Advanced Search
Anonymous User (login or join us)
Upload
See other formats

Full text of "A manual of photographic chemistry, theoretical and practical"

>^--Jlv 



^^^^-m 






f . '"^jy'--- ■■-7 • 






, -3t;:^--.4T^ 












T'^^^vtr 



./t:--^. .>^r.-/^ 



fi^ ^ ^^^-^- ... 1^- j¥ >if ^i^^- ^^ 






^^.- 



^ SEP 16 1932 
w y APR 23 



^^3 -3 '^3 

OCT 24 1-^ 



A MANUAL 

OF 

Photographic Chemistry, 

Theoretical and Practical 



REV. T. FREDERICK KARDWICH, M. A. 



BJIIVXH E»IXIOT»J, 



EDITED BY 

J. l^RAILL TAYLOR, 

Late Editor of the '' Photographic Times and A merican Photographer " Editor of the 

'^ British foiirnal of Photography : Corresponding Member of the Unperial 

Polytechnic Society of Russia ; Honorary Mejnber of the Edinburgh 

Photographic Society ^ the Association of Operative Photographers 

of New York^ the London Photographic Club, and the 

London and Provincial Photographers^ Association. 



NEW YORK : 
SCOVILL MANUFACTURING COMPANY 

W. Irving Adams, Agent. 
1886. 



Y 





^.A'tNSWw. 



"l>J^^^i^^v^\^^5■ 



A MANUAL 



PHOTOGRAPHIC CHEMISTRY. 






Digitized by the Internet Arciiive 

in 2010 witin funding from 

Boston Public Library 



http://www.archive.org/details/manualofphotogra1886hard 



CONTENTS. 



PAGE 

Historical Sketch of Early Photography 9 

■ CHAPTER I. 
Outlines of General Chemistry 15 

CHAPTER H. 
Vocabulary of Photographic Chemicals 32 

CHAPTER HI. 
The Salts of Silver Emplo}red in Photography 91 

CHAPTER IV. 
On the Development of an Invisible Image by Means of a Reducing 

Agent 103 

CHAPTER V. 
Fixing Agents 125 

CHAPTER VI. 
The Manufacture of Photographic Collodion 130 

CHAPTER VII. 
The Chemistry of the Nitrate Bath 152 

CHAPTER VIII. 
On Positive and Negative Collodion Photographs. . , 158 

CHAPTER IX. 
On the Theor}' of Positive Printing 84 

CHAPTER X. 
The Photographic Dark Room 219 

CHAPTER XL 
Formulas for Negative Solutions 221 



6 CONTENTS. 

CHAPTER XII. 
Manipulations of the Wet Collodion Processes 228 

CHAPTER XIII. 

PAGE 

The Positive Collodion Process on Glass or Metallic Plates 

(Ferrotypes) 234 

CHAPTER XIV.. 
The Negative Collodion Processes on Glass 236 

CHAPTER XV. 
Portraiture — Positive and Negative 245 

CHAPTER XVI. 
Photographic Enlargements 250 

CHAPTER XVII. 
The Practical Details of Photographic Printing 262 

CHAPTER XVIII. 
The Gelatine Emulsion Process , 302 

CHAPTER XIX. 
The Collodion Emulsion Process 317 

CHAPTER XX. 
Willis's Process for Engineers' Plans 319 

CHAPTER' XXI. 
Micro-Photography 322 

CHAPTER XXII. 
The Optics of Photography 325 

CHAPTER XXIII. 
On Binocular Vision and the Stereoscope 352 

CHAPTER XXIV. 

Chromotype or Lambert)^pe 361 

Appendix 368 



PREFACE 

.TO 

THE NINTH EDITION 



Since the publication of the previous editions of this 
Manual, two changes of importance have occurred ; the nomen- 
clature and atomic weights of chemical compounds have to a 
great extent been altered, and the collodion process has, 
especially in England, been largely supplanted by gelatine 
emulsion. 

It has, however, seemed judicious to the Editor of this edi- 
tion neither to entirely discard the old nomenclature in favor 
of the new, nor to assume the collodion process to be defunct. 
For while the chemist of the present day knows that — to ad- 
duce the case of a familiar body — the substance expressed by 
formula NagSgOs+S Aq. is Thio-Sulphate of Soda, there is 
scarcely one among thirty photographers or dealers in photo- 
graphic chemicals who is aware that this is the modern term 
to express Hyposulphite of Soda. Hence a work, intended to 
be of every-day use to photographers, would have its value im- 
paired by the exclusive use of terms as yet imperfectly un- 
derstood. 

Again, an edition of " Hardwich's Manual," devoid of the 
invaluable researches of Mr. Hardwich in the collodion pro- 
cess, would indeed be an anomaly, hence the Editor has restored 
as fully as possible the teachings of this savant on this topic, \ 
subject to the alteration of the chemical notation and equiva- 



8 PREFACE. 

lents to the requirements of the present time. It is worthy of 
notice that, notwithstanding the prevalent employment of gela- 
tine plates, the manufacture of collodion continues almost un- 
affected. The Editor is informed by one firm, presumably the 
most extensive collodion manufacturers in the world, that so 
far from their being any stagnation in that department, at no 
time in their business career has their been such a demand for 
collodion. 

The Chapters on the Optics of Photography, and on the 
Emulsion and other negative and printing processes, will be 
found to be brought up to the present time. 

To Mr. W. B. Bolton, Kr. E. W. Foxlee, and others, the 
Editor tenders his acknowledgments for information and ser- 
vices of a specialistic nature. 

. J. Traill Taylor, 

December, 1882. Editor. 




HISTORICAL SKETCH 



EARLY PHOTOGRAPHY. 



The art of photography, which has now attained such perfec- 
tion, and has become so popular amongst all classes, is one of 
comparatively recent introduction. 

The word Photography means literally " writing by means 
of Light ; " and it includes all processes by which any kind 
of picture can be obtained by the chemical agency of light, 
without reference to the nature of the sensitive surface upon 
which it acts. 

The philosophers of antiquity, although chemical changes 
due to the influence of light were continually passing before 
their eyes, do not appear to have directed tiieir attention to 
them. Some of the alchemists indeed noticed the fact that a 
substance termed by them " horn silver," which was chloride 
of silver which had undergone fusion, became hlaokened by 
exposure to light ; but their ideas on such subjects being of 
the most erroneous nature, nothing resulted from the dis- 
covery. 

The first philosophical examination of the decomposing ac- 
tion of light upon compounds containing silver was made by 
the illustrious ISweedish chemist, Scheele, in the year 1777. 
It was remarked by him that the maximum chemical or de- 
composing effect of the spectrum lay near the violet part and 
gradually diminished towards the red. He also attributed the 
blackening of chloride of silver by light to the liberation of 
chlorine and the formation of hydrochloric acid. Both these 
statements have been proved to be true. Scheele's experi- 
ments terminated here, but they do not seem to have attracted 
that attention which they deserved. In fact, they were 
looked upon as more curious than useful. 

Earliest AppliGation of these Facts to Purposes of Art. — 
The first attempts to render the blackening of silver salts by 



10 HISTORICAL SKETCH OF 

light available for artistic purposes were made by Wedg- 
wood and Davj, and published in 1802. But it is evident 
that the experiments must have been made several years 
previously, as Wedgwood had been dead several years before 
that date. A sheet of white paper or of white leather was 
saturated with a solution of nitrate of silver, and the shadow 
of the tigure intended to be copied projected upon it. Under 
these circumstances the part on which the shadow fell re- 
mained white, whilst the surrounding exposed parts gradually 
darkened under the influence of the sun's rays. 

Unfortunately, these and similar experiments, which appear- 
ed at the outset to promise well, were checked by the experi- 
mentalists being unable to discover any means of fixing the 
pictures, so as to render them indestructible by diffused light. 
The unchanged silver salt being permitted to remain in the 
white portions of the paper, naturally caused the proofs to 
blacken in every part, unless carefully preserved in the dark. 

It is singular that these distinguished philosophers, finding 
leather to be the most sensitive medium, did not hit on the 
cause — viz., taimin or gallic acid. 

The sensitive surfaces employed by Wedgwood and Davy 
could not be impressed in the camera, although, it would ap- 
pear, the attempt was made. Davy, however, succeeded in 
getting a faint impression in the solar microscope, where the 
image was much condensed in size. 

Wedgwood and Davy's plan of throwing the shadow of the 
object to be copied on to a medium sensitive to light forcibly 
reminds us of the origin of portrait painting as given by an 
ancient Roman writer : 

" Corinthi puella, capta amore juvenis, illo abeunte peregre, 
umbram ex facie ejus ad lucernam in pariete lineis circum- 
scripsit. Talis picturse fuit origo." 

Whether this was the origin of painting or not is a matter 
which cannot now be determined ; but we know, from authen- 
tic evidence, that this was the plan exactly adopted by Wedg- 
wood and Davy in their earlier experiments on photography. 
The only difference is that these eminent philosophers used 
'nature's pencil to delineate the object, and not a crayon 
worked by hand. 

Introduction of the Camera Ohscura, and other Imjprove- 
onents in Photography. — The " camera obscura," or darkened 
chamber, by means of which a luminous image of an object 
may be formed, was invented by Baptista Porto, of Padua ; 
but the preparations employed by Wedgwood were not suffi- 



EARLY PHOTOGRAPHY. 11 

ciently sensitive to be easily affected by the subdued light of 
that instrument. 

In the year 1814, however, twelve years subsequent to the 
publication of Wedgwood's paper, M. Niepce, of Chalons, 
having directed his attention to the subject, succeeded in 
perfecting a process in which the camera could be employed, 
although the sensibility was still so low that an exposure of 
some hours was required to produce the effect. 

In the process of M. Niepce, which was termed " helio- 
graphy," or " sun-drawing," the use of the silver salts was 
discarded, and a resinous substance, known as " bitumen of 
Judea," substituted. This resin was smeared on the surface 
of a metal plate, and exposed to the luminous action. The 
light in acting upon it so changed its properties, that it be- 
came insolulle in certain essential oils. Hence, on subsequent 
treatment with the oleao-inous solvent, the shadows dissolved 
away, and the lights were represented by the unaltered resin 
remaining on the plate. 

There are several specimens of Niepce's original process in 
the British Museum ; some of which also show his attempts 
at etching by the same process. 

The DisGoveries of M. Vaguerre. — MM. Niepce and Da- 
guerre appear at one time to have been associated as partners, 
for the purpose of mutually prosecuting their researches ; but 
it was not till after the death of the former, viz., in 1839, 
that the process named the Daguerreotype was given to the 
world. Daguerre was dissatisfied with the slowness of action 
of the bitumen sensitive surface, and directed his attention 
mainly to the use of the salts of silver, which are thus again 
brought before our notice. 

Even the earlier specimens of the Daguerreotj^pe, although 
far inferior to those subsequently produced, possessed a beauty 
M'hixih had not been attained b}^ any photographs prior to 
that time. 

The sensitive plates of Daguerre were prepared by exposing 
a silvered tablet to the action of the vapor of iodine, so as 
to form a layer of iodide of silver upon the surface. By a 
short exposure in the camera an effect was produced, not 
visible 'to the eye, but appearing when the plate was subjected 
to the vapor of mercury. This feature, viz., the production 
of a latent image upon iodide of silver, with its subsequent de- 
velopment by a chemical reagent, is one of the first importance. 
Its discovery at once reduced the time of taking a picture 
from hours to minutes, and promoted the utility of the art. 



12 HISTORICAL SKETCH OF 

Daguerre also succeeded in partiall^^ fixing his proofs by re- 
moval of the unaltered iodide of silver from the shadows ; but 
it was not till some years afterwards that attention was called 
to Herschel's researches on the hyposulphites. These contri- 
butions to chemical science were published in the course of 
the year 1821, in " The Edinburgh Philosophical Journal."' 
He not only investigated the chemical composition of the 
soluble hyposulphites, but also their power of dissolving the 
haloid salts of silver, within certain limits, when double salts 
were formed. These also he chemically formulated and de- 
scribed. Strange as it may appear, Herschel's discoveries 
escaped the attention of our early Daguerreotypists and 
workers by the Talbotype process. 

On a Means of Multiplying Photographic Impressions^ and 
other Discoveries of Mr. Fox Talbot. — The lirst communica- 
tion made to the Royal Society by Mr. Fox Talbot, in Jan- 
uary, 1839, related to the preparation of a more sensitive 
paper than had been previously known, and also to a method 
of fixing the pictures by common salt. It was directed that 
the paper should be first dipped in solution of chloride of 
sodium, and then in nitrate of silver. By proceeding in this 
way a white substance termed chloride of silver is formed, 
more sensitive to light than the nitrate of silver originally em- 
ployed by Wedgwood and Davy. The object is laid in con- 
tact with the prepared paper, and being exposed to light, a 
copy is obtained, which is negative., — id est, with the light and 
shade reversed. The discovery of a way of fixing these nega- 
tives was of great importance, as it not only rendered them 
unalterable by light, but it further allowed of their being 
used to obtain copies having the light and shade correct, or 
positive copies, which was effected by laying the negative over 
the second sheet of prepared sensitive paper, so as to allow 
the sun's light to pass through the transparent parts. Under 
these circumstances, when the negative is raised, a natural 
representation of the object is found below, the tints having 
been again reversed by the second operation. 

This production of a negative photograph, from which any 
number of positive copies may be obtained, is a cardinal point 
in Mr. Talbot's invention, and one of great importance.^ 

Another most important discovery was that of the existence 
of an invisible image impressed on the paper by a much short- 
er exposure to light than was necessary to produce a visible 
one, and capable of being afterwards rendered visible. Mr. 
Talbot took out a patent for a process of this kind in 1841. 



EARLY PHOTOGEAPHY. 13 

IVLr. Brayley had, however, as early as 1839, described in a lec- 
ture at the London Institution, a similar method communicated 
to him by the Rev. J. B. lieade. Mr. Talbot's process was, 
however, a decided advance on the former, in consequence of 
liis employing acetic acid to check action not due to light. In 
this process, a sheet of paper is first coated with iodide of silver 
by soaking it alternately in iodide of potassium and nitrate of 
silver ; it is then washed with solution of gallic acid, contain- 
ing nitrate of silver and acetic acid (sometimes termed gallo- 
nitr ate of silver), h J which the sensibility to light is greatly 
-augmented.- An exposure in the camera of some seconds or 
minutes, according to the brightness of the light, impresses an 
invisible image, which is brought out by treating the plate 
with a fresh ])ortion of the mixture of gallic acid, nitrate of 
silver, and acetic acid employed in exciting. 

On the Use of Glass Plates to Retain Sensitive Films. — The 
principal defects in the Calotype process are attributable to the 
•coarse and irregular structure of the fibre of paper, even when 
manufactured with the greatest care, and expressly for photo- 
graphic purposes. In consequence of this, the same amount 
of exquisite definition and sharpness of outline as that result- 
ing from the use of metal plates cannot be obtained. 

We are indebted to Sir John Herschel for the first employ- 
ment of glass plates to receive sensitive photographic films. 

The iodide of silver may be retained upon the glass by 
means of a layer of albumen or white of e^^, as proposed 
by M. Niepce de Saint- Yictor, nephew to the original dis- 
-coverer of the same name. 

A more important improvement still was the employment 
•of " collodion" for a similar purpose. 

Collodion is an ethereal solution of pyroxyline, a feebly 
explosive variety of gun-cotton. On evaporation, it leaves a 
transparent layer, resembling gold-beater's skin, which ad- 
heres to the glass with some tenacity. M. Le Grey, of 
Paris, originally suggested that this substance might perhaps 
I)e rendered available in photography, but our own country- 
man, the late Mr. F. S. Archer, was the first to carry the 
idea out practically. In a communication to "The Chemist," 
in the autumn of 1851, this gentleman gave a description of 
the collodion process much as it now stands; at the same 
time proposing the substitution of pyro-gdXWo, acid for the 
gallic acid previously employed in developing the image. 

At that period no idea could have been entertained of the 
•stimulus which this discovery would render to the progress of 



14 HISTOKICAL SKETCH OF EAKLY PHOTOGRAPHY. 

the art ; but experience lias now abundantly demonstrated 
that, as far as all qualities most desirable in a photographic 
process are concerned, except in extreme sensitiveness, none at 
present known can excel, or perhaps equal, the collodion pro- 
cess. 

Parallel with the above photographic discoveries were others 
in a different direction, and these, at the present day, are ex- 
ercising a vast influence on commercial photography. In the 
year 1839, Mr. Mungo Ponton announced to "The Royal 
Scottish Society of Arts " that bichromate of potash might be 
used to sensitize paper. The parts exposed to light became of 
a dark Orange tint, which was insoluble in water ; while the 
yellow color, not acted on by light, could be removed by 
washing in water. 

The full significance of this discovery was not appreciated 
till Mr. Fox Talbot afterwards showed that it was only in 
combination with solid matter that this salt was sensitive to 
light at all. In the year 1852 he took out a patent for the 
use of a bichromate and gelatine for a new process of engraving 
on steel. This is the foundation of almost all the photo-engrav- 
ing and photo-lithographic processes of the present day, be- 
sides others of still greater importance in photographic print- 
ing. 

In 1855, M. I'oitevin patented a carbon printing process 
founded on this principle. He dabbed over a sheet of paper 
a mixture of bichromate of potash, gum Arabic, and finally 
divided charcoal. When dry, it was exposed to light under a 
negative, and then placed in water. The parts unacted on by 
light were washed away, leaving the white paper exposed. 
The rest remained unchanged. This was the first carbon- 
printing process, by which, by judicious improvements, per- 
manent pictures of great beauty are now produced. 

When a large number of prints are required, the collotype 
and Woodbury processes, which have now reached a high state 
of perfection, afford the means of producing them suitable for 
book illustration. By these methods, the half-tones of a nega- 
tive are most perfectly rendered, while the rapidity of pro- 
duction is vastly increased. 

These remarks have been confined to what may be designated 
the early history of photography. 



OUTLINES 



GENERAL CHEMISTRY. 



CHAPTEE I. 



All substances with which we are acquainted, whether 
solid, liquid, or gaseous, are either elements or compounds of 
elements, or mixtures of elements, or mixtures of compounds. 
It is necessary, in the first place, to understand these terms 
clearly. 

Elements. 

An element is a substance which consists of one kind of 
matter only, or wiiich has never yet been decomposed or 
separated into two or more kinds of matter. The number of 
such simple substances at present known is 64 ; the names, 
symbols, and atomic weights are given in the following table, 
and it will be observed that both in respect of the number of 
elementary bodies given and the atomic weights or combining 
proportions, the table here given is more comprehensive than 
those in former editions of this work. 

In this table the most important elements are printed in a 
more conspicuous type than those of which less is known, or 
which are of less present importance. Those names to which 
an asterisk is affixed are non-metallic elements, the others not 
so distinguished are metals. 

The symbols attached to the elementary bodies are formed 
of the first letter of their Latin names, another letter being ad- 
ded to distinguish between two or more commencing with the 
same letter. 

The atomic or combining weights may represent grains, 
ounces, pounds, or any other weight. On account of hydrogen 
being the lightest of the known elementary bodies it is taken 
as the unit of the scale, and the numeral 1 affixed to it. 



16 



OUTLINES OF GENERAL CHEMISTRY. 



TABLE OF ELEMENT AEY BODIES WITH THEIR 
SYMBOLS AND ATOMIC WEIGHTS. 



Name. 



ALUMINIUM . . . . 

Antimony (Sti- 
bium 

Arsenic 

Barium 

Beryllium 

Bismuth 

Boron* 

BROMINE* 

Cadmium 

Caesium 

CALCIUM 

CARBON* 

Cerium 

CHLORINE* 

Chromium 

Cobalt 

COPPER (Cu- 
prum) 

Davyum 

Didymium 

Erbium 

FLUORINE* 

Gallium 

Gold (Aurum).. . 

HYDROGEN* 

Indium 

IODINE* 

Iridium 

IRON (Ferrum) . . 

Lanthanum 

LEAD (Plumbum) 

Lithium 

Magnesium 

MANGANESE... 
MERCURY (Hy- 
drargyrum 



Sym- 
bol. 



Al 

Sb 

As 

Ba 

Be 

Bi 

B 

Br 

Cd 

Cs 

Ca 

C 

Ce 

CI 

Cr 

Co 

Cu 

Da 

D 

E 

F 

Ga 

Au 

H 

In 

I 

Ir 

Fe 

La 

Pb 

Li 

Mg 

Mn 

Hg 



Atomic 
Weight. 



27.4 

122 

75 
137 

9.4 
210 

11 

80 
112 
133 

40 

12 

92 

35..5 

52.2 

58.8 

63.4 

95 
112.6 

19 

68 
197 
1 
113.4 
127 
198 

56 

93.6 
207 
7 

24 

55 

200 



Name. 



Molybdenum . . . 

Nickel 

Niobium 

NITROGEN*. . . . 
Osmium 

OXYGEN* 

Palladium 

PHOSPHOROUS- 
PLATINUM- - - . 

POTASSIUM 
(Kalium) 

Rhodium 

Rubidium 

Ruthenium 

Selenium*. . . 

SILICIUM* 

SILVER (Argen 
gentum 

SODIUM (Na- 
trium 

Strontium . . . . , 

SULPHUR* 

Tantalum , 

Tellurium* , 

Thallium 

Thorinum 

TIN (Stannum)., 

Titanium 

Tungsten, or 

Wolfram 

Uranium 

Vanadium 

Yttrium 

ZINC- 

Zirconium 



Sym- 
bol. 



Mo 

Ni 

Nb 

N 

Os 

O 

Pd 

P 

Pt 

K 
Rh 
Rb 
Ru 

Se 
Si 

Ag 

Na 

Sr 

S 

Ta 

Te 

Tl 

Th 

Sn 

Ti 

W 

U 

V 

Y 

Zn 

Zr 



Atomic 
Weight. 



96 

58.8 

94 

14 
199.2 

16 
106.6 

31 
197.4 

39.1 

104.4 
85.4 

104.4 
79.4 
28 

108 

23 

87.6 

32 
182 
128 
204 
235 
118 

50 

184 

240 
51.2 
61.7 
65.2 
89.6 



SYMBOLS. 



17 



TABLE OF SYMBOLS OF THE MORE IMPOETANT 
COMPOUNDS USED IN PHOTOGRAPHY. 



Name. 



Symbol. 



Acid, Acetic (Cryst.) 

" Citric 

" Formic 

" Gallic 

" Hydriodic 

" Hydrobromic 

" Hydrochloric 

" Hydrocyanic 

** Hydrosulphuric 

(Sulph. Hydro) 

" Nitric 

" Pyrogallic 

" Sulphuric 

" Tannic 

Alcohol 

Ammonical Gas 

Ammonium, Bromide 

Chloride 

Iodide 

" Nitrate 

" Sulphydrate of. . . 

" Sulphocyanide of 

Barium, Chloride (Cryst.) 

Baryta, Nitrate of 

Benzole 

Chloroform 

Cadmium, Bromide (Commer.) 

'' Iodide 

Calcium Bromide (Cryst.) .... 

Chloride 

Ether 

Gold, Terchloride 

Iron, Perchloride 

Iodide 

" Nitrate 



H.CaHsOa 60 

H3,C6H507 + H20 210 

H,CH02 46 

H,C7H505 170 

HI 128 

HBr 81 

HCl 36.5 

HCN 37 

H2S 34 

H.NO3 63 

HsCeHsOs 126 

HSOi 98 

C14H10O9 322 

C^HeO 46 

NH3.... 17 

NHiBr 98 

NHiCl 53.5 

NHJ 145 

NHi,N03 80 

NHi,HS 51 

NH4,CNS 76 

Ba,Cl2 + 2H20 244 

Ba,(N03)2 261 

CgHe 78 

CH,Cl3 119.5 

Cd,Br2 + 4H20 344 

Cdia 366 

CaBr, + 4H20 272 

CaCla Ill 

CiHioO 74 

AuClg 302.5 

FeaCle 325 

Fela 310 

Fe(N03)2 + 6H20 288 



18 



OUTLINES OF GENERAL CHEMISTRY. 



TABLE OF SYMBOLS, ET^c.—Contimied. 



Name. 



Symbol. 



of 



Iron, Sulphate 

" Double Sulphate 

Ammonia 

Lead, Acetate (Cryst.) 

" Nitrate 

Lithium, Iodide 

Lithium, Bromide 

Mercury, Chloride 

(Corrosive Sublimate.) 

Mercury, Subchloride 

(Calomel) 

Potassium, Bichromate 

" Carbonate 

** Hydrate 

" Nitrate 

" Bromide 

" Chloride 

" Cyanide 

" Iodide 

Silver, Acetate 

" Bromide 

" Carbonate 

Chloride 

" Hyposulphite 

Iodide 

" Nitrate 

" Oxide 

Sulphide 

Sodium, Acetate (Cryst.). . . . 
" Carbonate (Cryst.). . 

" Hyposulphite (Cryst 

" Nitrate 

" Bromide 

Chloride 

" Iodide 

Uranium, Nitrate 

Zinc, Bromide 

" Chlorate 



FeSO. + TH^HO 278 

FeSO,,(NH4)2SO, + 6H20. 392 

Pb,(C2H305)+2H20 343 

Pb,(N03)' 331 

LilGHaO 242 

LiBr 87 

HgCl2 271 

Hg^Cla 471 

K2Cr207 294.6 

K2CO3 138.2 

KOH 56.1 

KNO3 101.1 

KBr 119.1 

KCl 74.6 

KCN 65.1 

KI 166.1 

AgCaHsOa 167 

AgBr 188 

AgoCOa . 276 

AgCl 143.5 

Ag^SijOs 328 

Agl 235 

AgNOs 170 

AggO 232 

Ag^S 248 

NaCaHsO^ + eHeO 190 

NaaCOs + lOH^O 286 

NaaSaOs + SH^O 248 

NaNOa 85 

NaBr 103 

NaCl 58.5 

Nal 150 

(UrOe),(N03)' + 6H20.... 384 

ZnBr^ 225.2 

ZnCIa 136.2 



compounds and mixtures. 19 

Compounds and Mixtures. 

These elementary substances are capable of uniting with each 
other to form compounds, which differ from their elements in 
appearance and properties, the more completely that the latter 
were opposed to each other in their properties. Thus the gaseous 
element chlorine is capable of combining with the metal 
sodium, to form with it the well-known solid, common salt, in 
which neither a gas or a metal can be detected by the senses. 
The black solid, iodine, unites with the metal potassium, to 
form the white compound iodide of potassium. The two gases 
oxygen and hydrogen, form by their union, the liquid, water. 
Chemical combination is often attended with heat and light, the 
union of bodies with the oxygen of the air, when accompanied 
with these phenomena, being termed combustion. 

Mixture differs entirely from Chemical combination^ in that 
it leaves the properties of the ingredients comparatively un- 
changed and perceptible to the senses, it is not attended with 
heat, excepting that which may be due to the friction used, and 
very simple means usually serve to separate the ingredients 
again. Thus, gunpowder is an example of the most perfect 
mixture of the three ingredients, charcoal, nitrate of potash, 
and sulphur ; yet the sense of taste will detect the nitre, the 
sulphur may be discovered by the smell when rubbed, and the 
eye perceives the color of the charcoal. Sulphur and finely 
divided iron might be so intimately mixed, that the two in- 
gredients could not be separately distinguished by the eye, but 
the iron might still be separated by a magnet from the sulphur ; 
if, however, true combination were brought about by the ap- 
plication of a gentle heat, a great increase of heat is suddenly 
perceived, and if the materials were in the right proportions, 
the magnet would no longer discover any iron in the resulting 
compound. Hydrogen and oxygen gases may be mixed, and 
they, will remain gases still, but when combined the gaseous 
character is lost, and the liquid, water, results. 

Of the compounds produced by the union of two elements, 
the class of oxides, formed by the combination of oxygen with 
another element, is the most important. These oxides form 
three important classes, namely, bases, acids, and neutral oxides, 
each of which must be separately considered. 

Bases. 

The basic oxides are found only among the metallic oxides, 
although the whole of the metals do not furnish basic oxides. 



20 OUTLINES OF GENERAL CHEMISTRY. 

The most perfect examples of bases are the oxides of potassium, 
sodium, lithium, barium, calcium, strontium, magnesium ; the 
oxides of the first three, potassium, sodium, and lithium, 
known as potash, soda, and lithia, are distinguished 
from other bases by the term " alkali ;" the oxides of the 
other four are known as "alkaline" earths. The alkalies 
and alkaline earths exhibit all the true characteristics of bases 
in perfection — viz., causticity and corrosiveness, and the power 
of uniting with another class of compounds, the acids, equally 
caustic and corrosive, and of producing compounds compara- 
tively inactive and harmless, they possess also the property of 
changing the red of litmus to blue, and the yellow of turmeric 
to brown. The property of litmus and tumeric in changing 
color with bases and acids has caused them to be used for 
"test-papers," which serve to detect an exceedingly minute 
amount of one of the stronger bases, or acids in a free state. 
The other metallic oxides termed bases, exhibit the basic char- 
acter in very variable degrees, some, such as the oxides of 
silver and lead, " neutralize" the strongest acids perfectly, so 
that the product no longer afiects test-paper ; others, such as 
the oxides of zinc and iron, neutrahze acids, but imperfectly, 
so that the products of the combination always show an acid 
reaction with test paper. 

Acids. 

The most important of the acid oxides are those of non- 
metallic elements ; only a few of the metals, as arsenic, chro- 
mium, and manganese, furnishing acid oxides. Some metals, 
as chromium and manganese, have both basic and acid oxides; 
when this is the case, the acid oxide always contains the 
most oxygen. 

Taking the oxides of nitrogen and sulphur known as nitric 
and sulphuric acids as types, the characteristics of acids may 
be stated to be, intense sourness and corrosiveness, the power 
of "neutralizing" the strongest bases, so that the product no 
longer affects test-paper, and the property of changing blue 
litmus to red, even when extremely dilute. The acid oxides, 
like the bases, possess these characters in very different degrees, 
the weaker bases and acids approaching each other in character, 
— and in the same proportion, having little tendency to combine. 

The stronger oxygen acids always retain, even in their con- 
centrated state, a certain amount of water, which is essential 
to them. If deprived of this, they form what are termed 
" anhydrides,"which are essentially different from acids. 



ACIDS. 21 

The modern theory of the oxygen acids is to consider them 
as salts of hydrogen in which the hydrogen is replaced by a 
metal in combination. Thns — 

HNOg+KHO^KNOa + HgO. 

Hydrogen Acids. — While speaking of acids, it may be as 
well to mention that besides the acids above alluded to, which 
are oxides, there are others which contain no oxygen, but are 
compounds of hydrogen with certain elements, chiefly chlorine, 
bromine, iodine, fluorine, and sulphur, and with certain com- 
pounds which perform many of the functions of these ele- 
ments. These acids are distinguished by the prefix hydro-, 
the rest of the name being derived from the other element 
of the acid ; thus the compound of chlorine and hydrogen is 
termed hydrochloric acid, that of hydrogen and iodine, hy- 
driodic acid. Most of these hydrogen acids are gaseous bodies, 
and the liquids known under their names are solutions of the 
same in water. 

jNeutkal Oxides. 

Besides forming acid and basic oxides, oxygen also unites in 
a few cases with elements to form neutral bodies which have 
no tendency to unite with either acids or bases. Such are two 
of the oxides of nitrogen, the oxide of carbon, and the black 
oxide of manganese. 

Salts. 

The combination of an acid with a base produces what is 
termed a salt. This class of compounds is a very large and 
important one, and includes many substances that would not 
ordinarily be recognized under that term. When the afiinities 
of the acid and base of the salt are about equal, as is the case, for 
example, with sulphuric acid and potash in sulphate of potash, 
the gait has no action on test-paper. In many cases however 
the reaction of the acid or base preponderates ; thus sulphate of 
iron, a compound of a powerful acid with the weak base oxide 
of iron, always has an acid reaction ; carbonate of soda, on the 
other hand, consists of a strong base and a feeble acid, the cai*^ 
bonic, and therefore is always alkaline to test-paper. Both of 
these salts, though far from neutral in reaction., are yet neutral 
in composition. 

Salts, derived from oxygen acids, always contain the whole 
of the acid and base, but with hydrogen acids the case is diff'er- 
ent ; in these the hydrogen of the acid unites with the oxy- 



22 OUTLINES OF GENERAL CHEMISTKY. 

gen of the base to form water, which separates on evaporation 
to dryness, and the other element of the acid unites with the 
metal of the base to form what is termed a " haloid salt," a 
name derived from aki, a\o5, (Halos), sea salt, because common 
salt, chloride of sodium, is a familiar example. This compound 
may be formed by the union of hydrochloric acid and soda 
(oxide of sodium), and would be termed hydrochlorate of soda 
if it retained the acid and base entire, but as each is decom- 
posed, one giving up its hydrogen and the other its oxygen to 
form water, which separates, the chlorine of the acid and 
sodium of the base alone remain, and the product of their 
combination is termed chloride of sodium, which contains no 
longer either acid or base, and yet is a true salt. Examples of 
such salts are iodides, bromides, fluorides, and chlorides of 
the various metals. The haloid salts however, when decom- 
posed, yield products similar to the oxyacid salts. For in- 
stance, if iodide of potassium be dissolved in water, and dilute 
sulphuric acid added, this acid, being pow^erful in its chemical 
affinities, tends to appropriate to itself the alkali ; it does not 
however remove potassium and liberate iodine^ but takes the 
oxide of potassium and sets free hydriodic acid. In other 
words, as an atom of water is produced during the formation 
of a hydracid salt, so are the elements of water separated in. 
the decomposition of a hydracid salt. The reaction of dilute 
sulphuric acid upon iodide of potassium may be stated thus : 

Sulphuric acid//MJ (iodine potassium) //?<j (hydrogen oxygen) 
equals (sulphuric acid, oxygen potassium) or sulphate of potash, 
and (h)'drogen iodine) or h3'driodic acid. 

All metallic salts with oxygen acids contain the metal in the 
state of oxide, although in the name of the salt the word oxide is 
omitted ; thus sulphate of iron is strictly sulphate of oxide of 
iron, and nitrate of silver, nitrate of oxide of silver. The 
oxides of the alkaline and earthy metals, being known long 
before their elementary metals were discovered, have each a 
name more familiar than that of the metals, hence in the salts 
with oxygen acids the former name is employed rather than 
the latter ; thus nitrate of lime is spoken of, rather than ni- 
trate of oxide of calciuTn. In the haloid salts of these ele- 
ments, the name of the metal is necessarily made use of ; thus ' 
hydrochloric acid and lim,e form chloride of calcium (and 
water). 

Since acids do not combine with metals, but only with their 
oxides, every metal wdien dissolving in an acid obtains oxygen, 
either from the water or from the acid, or, in the case of hy- 



ATOMIC THEORY. 23 

dracids, displaces the hydrogen of the acid, and combines with 
the remaining element of the acid. Thus, when zinc or iron 
dissolves in dilnte snlphnric acid, water is decomposed, the 
oxygen combining with the metals to form oxides, which nnite 
with the acid, and the other element of the water, hydrogen, 
escapes with effervescence. When the same metals dissolve in 
a hydrogen acid, as hydrochloric, hydrogen is evolved from the 
acid, and the remaining element, chlorine, forms with the zinc 
or iron, chloride of zinc or iron. On the other hand, when 
silver dissolves in nitric acid, the oxygen required to form 
oxide of silver is derived from the acid, which is thus reduced 
to a lower oxide of nitrogen, which escapes with effervescence. 

Laws of Combination by Weight. 

These are four in numl;)er, and they are of much im- 
portance. 

1. Law of Definite Proportion. — " Every chemical com- 
pound has a perfectly definite composition, and the same com- 
pound, from whatever source derived, always has the same 
elements in the same proportions." 

2. Law of Multiple ProjMrtions. — "If one body can com- 
bine with another in more than one proportion, the other pro- 
portions are either double, treble, etc., of the first, or bear 
some nearly equally simple proportions." 

3. Law of Equivalent Proportions. — " If certain bodies, as 
A, B, and C, unite each separately with another, X, the 
proportions in which they combine with X to form their 
simplest compounds, are the same (or some simple proportion 
of the same) in which they combine with each other to form 
their simplest compounds." 

4. Laio of Combining Numbers of Compoxinds. — The com- 
bining number, or atomic weight, of a compound is the sum 
of the combining numbers of its constituents. 

Atomic Theory. 

The remarkable facts stated in these laws have led to a con- 
jecture or theory, termed the^ atomic theory, that however 
minutely divisible matter may be, it is not practically in- 
finitely divisible, but that it consists of ultimate particles which 
never are divided in all the changes which matter undergoes, 
and which are therefore termed atoms, from atofxoi (uncut). 
It is supposed that the simplest combination of two elements 
consist of 1 atom of one united to 1 atom of the other, and as 



24 OUTLINES OF GENERAL CHEMISTRY. 

the numbers attached to the elements represent the proportions 
of the simplest combinations, they also represent the propor- 
tional weights of the atoms of each of the elements, or their 
atomic weights. 

In describing the composition of a substance, it is common 
to speak of it as consisting of one atom of one element com- 
bined with one atom (or more) of another element, whatever 
the quantity of the substance may be, meaning that the min- 
utest particle or atom of it has this composition. 

Practical Application of the Laias of Comhination. — The 
utility of being acquainted with the law of combining propor- 
tions is obvious when their nature is understood. As bodies 
both unite with and replace each other in equivalents, a simple 
calculation shows at once how much of each element or com- 
pound will be required in a given reaction. Thus, supposing 
it is required to convert' 100 grains of nitrate of silver into 
chloride of silver, the weight of chloride of sodium which will 
be necessary is deduced thus : — one equivalent, or 170 parts, 
of nitrate of silver is decomposed by an equivalent, or 58.5 
parts, of chloride of sodium. Therefore, as 170 : 58.5 : : 100 
: 34.4 ; that is, 34.4 grains of salt will precipitate, in the state 
of chloride, the whole of the silver contained in 100 grains of 
nitrate. 

So again, in order to form the iodide of silver, what are the 
proportions in which the two salts should be mixed ? The 
equivalent of iodide of potassium is 16'), and that of nitrate of 
silver is 170. These numbers so nearly correspond, that it is 
common to direct that equal weights of the two salts should be 
taken. 

One more illustration will suffice. Supposing it be re- 
quired to form 20 grains of iodide of silver, how much iodide 
of potassium and nitrate of silver must be used ? One equiv- 
alent, or 166 parts, of iodide of potassium, will yield an equiv- 
alent, or 235 parts, of iodide of silver ; therefore, as 235 : 
166 : : 20 : 14.1. Hence, if 14.1 grains of the iodide of po- 
tassium be dissolved in water, and an equivalent quantity — viz., 
14.5 grains — of the nitrate of silver added, the yellow precipi- 
tate, when washed and dried, will weigh precisely 20 grains. 

Nomenclature of Compounds. 

Compounds of Two Elements. — A compoimd of two elements 
is distinguished by the name of the first terminating in -ide ; 
thus FeO = o^ide of iron, NaCl = chloride of sodium, Kl = 
iodide of potassium. The same termination is also used with 



NOMENCLATURE OF COMPOUNDS. 25 

compound bodies wliich possess neither basic nor acid charac- 
ters when the J unite with each other or with an element ; thus 
cyanogen, ON, a compound neither acid nor basic, unites with 
potassium, forming cyanide of potassium, K,CN or KCy. So 
also the salts produced by the combination of hydrogen acids 
with bases, since they are compounds of only two elements, in 
consequence of the separation of hydrogen and oxygen as 
water, are named in the same way. Salts of this kind may be 
produced eitlier by the union of two elements, or by the union 
of an acid and base. 

KHO + HI = KI +H2O 
NaHO + HCl = NaCl + H^O. 
KHO + HCy = KCy + H2O. 

The combinations of sulphur, carbon, and phosphorus, with 
other, elements used to be distinguished by the termination 
-uret^ as FeS = sulph^^i^^^ of iron, but the termination -ide is 
now generally used with all hinary compounds, or compounds 
of two elements. Phosphide, or sulphide, or carbide of iron, 
are now the terms used instead of phosphuret, sulphuret, and 
carburet of iron. 

When one element combines with another in more than one 
proportion, it was, under the old system of nomenclature, cus- 
tomary to distinguish the compound which consists of an 
atom of each by the prehx proto-, as FeClg = protochiovi^e of 
iron, ISTgO = protoy^i^e of nitrogen ; the compound of three 
atoms of the lirst-named* with two atoms of the second ele- 
ment has the prefix sesqtd-, as FegCle = sesqtdohloYi^e of iron ; 
the prefix hi- or hin- signifies 2 atoms of the first to 1 of the 
second, iaxdiper-, the compound, not acid, which contains the 
largest amount of the same element ; thus, MnOj is the binox- 
ide and also the peroxide of manganese, as it is the highest 
non-acid oxide of that metal. 

Compounds which contained more than one atom of the 
seco7id-named element to one of the first, were classified as 
sw^compounds; the quantity being defined by other prefixes, as 
di- and tri-, wliich were the reverse of hi- and ter-, and signi- 
fying one atom of the first-named to two and three respectively 
of the other element. 

Under the new system the Greek prefixes mono-, di-, tri-, 
tetra-, etc., alone are employed to distinguish between the 



* First and second relates to the order of the elements in the names of the 
compounds, and not to their order in the formulas. 



26 OUTLINES OF GENERAL CHEMI8TEY. 

different conipoimds. Thus, silver monoxide (AggO) repre- 
sents the combination between silver and the lowest quantity 
of oxygen. 

Acids and Salts. — If but one acid is known as resulting 
from the union of two elements, its name ends in ic ; this is 
always the case with the hydrogen acids. In the case of oxy- 
gen acids, when there is a series of acids produced by the com- 
bination of an element with oxygen, that which contains the 
largest amount of oxygen terminates in -ic, the next which 
contains less ends in -ous, and if there be a third with still less 
the prefix hypo- is used ; thus, bearing in mind what has been 
previously said, 11^03 = nitric acid, HNO2 = nitrow* acid; 
H2SO4 = sulphuric acid ; II oS O3 = sulphurous acid, and HgSOg 
= hypo^v\^h.\\\'ous acid. 

Sometimes after distinguishing tlie highest known acid of a 
series with the termination -^c, another acid is discovered with 
still more oxygen, in this case the new compound has the pre- 
fix jper- attached ; thus, after HCIO3 = chloric acid was named, 
another acid, HCIO^, was discovered, and named j?erchloric 
acid. 

In the name of a salt the -io is changed into -ate, and -ous 
into -ite ; thus, sulphur*6* acid forms sulph«fe6', and sulphnrcw.s 
acid forms sulph?'^«s ; the prefixes of the acids, as hypo- or per-, 
being retained in the salt, as A2/j>osulphite of soda, jf?6/'mangan- 
ate of potash. 

A chemically neutral salt is one which has as many atoms of 
acid as there are atoms of oxygen in the base, thus Fe,S04 and 
FogOs, 3SO3 are each chemically neutral (though from the 
weakness of their bases, quite acid to test-paper) ; such salts 
were most correctly distinguished as sulphate of the protoxide, 
and sulphate of the sesquioxide of iron, the name of the oxide 
indicating the number of atoms of acid they must contain ; 
commonly, however, the prefix which distinguishes the oxide 
is transferred to the acid, and the above-named salts are thus 
called j!?r<9^osulphate and sesqui&vl^h&tQ of iron. But these 
and similar salts are now more commonly described as ferrous 
sulphate and ferric sulphate, or in other words as sulphates 
respectively of the ferrous and ferric oxides. Prefixes to the 
acid in a salt ought properly to be confined to those cases in 
which there are not the same number of atoms of acid as there 
are of oxygen in the base, the prefixes sesqui-, bi-, ter-, di-, 
and tri- having the same meanings as in the case of 
compounds of two elements, the three first being applied to • 
Ghemically acid, and the two latter to a chemically basic salts. 



CHEMICAL CHANGES. 27 

In salts derived from hydrogen acids, since they are binary 
compounds, the prelix of the oxide is necessarily transferred 
to the residue of the acid ; thus hydrochloric acid forms with 
sesquioxide of iron, sesquiohloride of Iron (and water). 

FeA + 61101 = FcCle + SH^O. 

Chemical Changes. 

Chemical changes, whether they be the result of combina- 
tion or of decomposition, or of interchange of elements be- 
tween compounds, are often attended with striking phenomena, 
such as the evolution of light and heat, or with remarkable 
alterations of condition, as from gas to liquid, or from liquid 
to solid, etc. ; but the changes most familiar to the photog- 
rapher and the analyst are those of color and of solubility. With 
regard to the latter, it is well here to explain one or two terms. 
A body is said to dissolve in a liquid, when it disappears in the 
liquid, leaving the latter transpar'ent, even though it may be 
colored. Two transparent solutions, when mixed, often pro- 
duce 2i precipitate, a term which is applied to insoluble matter 
suddenly appearing in a liquid previously clear, and rendering 
it opaque and turbid. 

Precipitates can be separated either by decantation or filtra- 
tion. The iirst method, decantation, is allowing the precipi- 
tate to fall by its own weight to the bottom of tlie liquid, and 
then pouring the latter off. Filtration is affected by pouring 
the whole liquid and precipitate on a piece of porous paper, 
folded into a conical form, termed 2i filter, and supported in a 
funnel y the liquid passes through the paper in a transparent 
state, and the precipitate remains upon it. But chemical 
changes are here referred to chiefly for the purpose of point- 
ing out the varieties of internal change which bodies may un- 
dergo when brought into contact with each other ; these 
internal changes manifesting themselves often in the visible 
phenomena above alluded to. Alterations of composition may 
usually be referred to under one of the following heads : — 

1. Simple and direct union of two elements. — Thus, sulphur 
combines with the oxygen of the atmosphere when heated to 
a certain point, the union being known as " burning," and the 
product is sulphurous acid, II2SO3. The element iron " rusts " 
by combining with oxygen. 

2. Union of compounds with elements. — Thus, sulphurous 
acid, II2SO3, by exposure to air can combine with more oxygen. 



28 OUTLINES OF GKNEKAL CHEMISTRY. • 

H2S03 + = 1X2804 = sulphuric acid. Protoxide of iron, FeO, 
by exposure to air becomes Fe^Oa. 

2FeO + = Fe203. 

3. Union of compounds with each other, as in the forma- 
tion of salts with oxygen acids. 

KHO + HS04==KS0i + 1X20 = sulphate of potash. 

Two salts sometimes unite together to form a double salt. 
Alum is an example, it is a double salt of sulphate of alumina 
and sulphate of potash. 

The double chloride of gold and sodium is another example, 
AuC]3+ NaCl + 4H0. 

4. Displacement, or single decomposition, is where one ele- 
ment or compound displaces another element or compound 
from its state of combination. 

Thus chlorine acting on iodide of potassium, immediately 
takes the potassium, and sets iodine free, a change which is 
rendered evident from the black color of the iodide. KI + 
C1 = KC1 + I. 

One metal often precipitates another from its solution by 
displacing it ; thus zinc placed in a solution of acetate of lead 
combines with the acetic acid and oxygen of the oxide of lead, 
and is dissolved, while the latter is thrown down in an arbor- 
escent form, known as the " lead tree." 

A piece of metallic copper, placed in a solution of nitrate of 
silver, will speedily throw down metallic silver, and form ni 
trate of copper. 

One oxide is often used to separate another oxide ; the oxide 
of silver is obtained by adding a solution of potash to a solu- 
tion of nitrate of silver. 

2AgN03 + 2KH0 = AgaO + 2KNO3 + H^O. 

5. Double decomposition is the mutual interchange of the 
atoms of two compounds ; in the case of solutions this is often 
attended with precipitation ; thus, in the preparation of nitrate 
of iron from nitrate of baryta and sulphate of iron, there is a 
mutual interchange, resulting in nitrate of iron and sulphate of 
baryta, which is precipitated. 

In the sensitizing of salted paper with nitrate of silver, 
chloride of silver is precipitated in the paper, the change being 
Ag,N03 + NaCl = Na,N03 + AgCl. 



ON THE CHEMISTBY OF ORGANIC SUBSTANCES. ' 29 

6. Substitution. — Tins is only a variety of displacement or 
of double decomposition, but the term is used chiefly in cases 
where one of the compounds that has undergone the displace- 
ment is little, if at all, altered in appearance, as in the case of 
cotton transformed into gun-cotton. Here a certain number of 
atoms of hydrogen have been displaced by peroxide of nitrogen, 
but the appearance of the cotton is little, if at all, changed. 

7. Decomposition, into simpler compounds or elements, as 
of chloride of silver by light, which resolves it into subchloride 
(?) of silver and chlorine ; strong nitric acid by light is decom- 
posed into NOg and O. Chlorate and iodate of potash by heat 
are resolved into oxygen, and chloride and iodide of pot- 



assium. 



K,C103 = KCl + 03. 
K,I03 = KI + 03. 

Several of the compounds of gold, silver, platinum, and 
mercury are entirely decomposed into their elements by heat. 

On the Chemistry of Organic Substances. 

The term organic is applied to substances produced either 
by vegetables or animals, or which are obtained by chemical 
processes from them ; thus wood, and acetic acid procured by 
its distillation, and sugar, with the alcohol derived from it by 
fermentation, are organic substances. 

All organic bodies are compounds of carbon, and the class 
embraces a great variety of products, which, like inorganic 
bodies, include neutral, acid, basic and saline compounds. 

The organic acids are numerous, such as acetic acid, tartaric, 
citric, and a variety of others. 

The neutral substances cannot easily be assimilated to any 
class of inorganic compounds ; as examples, take starch, sugar, 
lignrne, etc. 

The hases are also a large class. Morphia, obtained from 
opium ; quinia, from quinine ; nicotine, from tobacco, are 
illustrations. 

Besides these, there are large classes of compounds known 
as alcohols, ethers, aldehyds, etc., to which there is nothing 
analogous in inorganic chemistry. 

Alcohols are carbon compounds, which by union with acids 
form neutral bodies, termed ether, water being separated. 

Ethers are either alcohol, + acid minus water, or 2 alcohol 
minus 2 water. 



30 OUTLINES OF GENERAL CHEMISTRY. 

Aldehyds are alcohols tninus hydrogen, and by the absorp- 
tion of oxygen they form acids. 

Composition qf Organic and Inorganic Bodies Contrasted. — 
There are more than fifty elementary substances found in the 
inorganic kingdom, but onlj^our, commonly speaking, in the 
organic : these four are carbon, hydrogen, nitrogen, and 
oxygen. 

Some organic bodies — oil of turpentine, naphtha, etc., — con- 
tain only carbon and hydrogen ; many others, such as gum, 
sugar, alcohols, fats, vegetable acids, carbon, hydrogen and 
oxygen. The nitrogenous bodies, so called, contain nitrogen 
in addition to the other elements : such are albumen, caseine, 
gelatine, and all natural organic bases ; sulphur and phosphorus 
are also present in many of the nitrogenous bodies, but only 
to a small extent. 

Organic substances, although simple as regards the number 
of elements involved in their formation, are often highly com- 
plex in the arrangement of the atoms ; this may be illustrated 
by the following formulas : 

Starch CeHioOs. 

Cane sugar CizHzjOn. 

Grape sugar .... CeHijOe. 

Inorganic bodies, as already shown, unite in pairs — two 
elements join to form a binary compound ; two binary com- 
pounds produce a salt ; two salts associated together form a 
double salt. With organic bodies, however, the arrangement 
is often different — the elementary atoms are all grouped equally 
in one compound atom, which is highly complex in structure, 
and cannot be split up into binary products. 

Observe also, as characteristic of organic chemistry, the ap- 
parent similarity in composition between bodies which differ 
widely in properties. As examples take lignine, or cotton 
fibre, and starch — each of which contains the same percentage 
of carbon, hydrogen and oxygen. 

Mode of Distinguishing Between Organic and Inor- 
ganic Matter. — A simple means of doing this is as follows : — 
Place the suspected substance upon a piece of platinum-foil, 
and heat it to redness with a spirit-lamp ; if it first blackens, 
and then burns completely away, it is probably of organic 
origin. This test depends upon the fact, that all organic bodies 
contain carbon, and that their other constituent elements are 
either themselves volatile, or capable of forming volatile com- 
binations with oxygen. Inorganic substances, on the other 



ON THE CHEMISTRY OF ORGANIC SUBSTANCES. 31 

hand, are often unaffected by heat, or, if volatile, are dissipa- 
ted without previous charring. 

The action of heat upon organic matter may be illustrated 
by the combustion of coal or wood in an ordinary furnace : — 
first, an escape of carbon and hydrogen, united in the form of 
volatile gaseous matter, takes place, leaving behind a black 
cinder, which consists of carbon and inorganic matter combined ; 
afterwards this carbon burns away into carbonic acid, and a 
gray ash is left, which is composed of inorganic salts, and is 
indestructible by heat. 

This test is of course not applicable to organic bodies volatile 
without decomposition, as alcohol, ether, camphor, etc. ; but 
these also are generally not the varieties of organic matter 
which trouble the photographer. 



CHAPTEK 11. 

YOCABULARY OF PhOTOGKAPHIC ChEMICALS.* 

ACETIC ACID. 

Formula of Glacial Acid, HCaHsOi. 

Acetic acid is a product of tlie oxidation of alcohol, C2II6O. 
Alcohol when perfectly pure, is not affected by exposure to 
air ; but if it be diluted, and a portion of yeast be added, it 
soon acts as ^ferment, and causes the spirit to unite with oxy- 
gen derived from the atmosphere, and so to become sour f I'om 
formation of acetic acid, or " vinegar." 

Acetic acid is also produced on a large scale by heating wood 
in close vessels ; a substance distils over which is acetic acid 
contaminated with empyreumatic and tarry matter; it is 
termed pyroligneous acid, and is much used in commerce. 

The most concentrated acetic acid may be obtained by neu- 
tralizing common vinegar with carbonate of soda, and crystal- 
lizing out the acetate of soda so formed ; the acetate must then 
be deprived of its water of crystallization, and fused at a gentle 
heat. After cooling, 82 parts of the salt are to be distilled 
with 98 of strong sulphuric acid, which removes the soda and 
liberates acetic acid, the acetic acid being volatile, distils over, 
and may be condensed. 

Properties of Acetic Acid. — The strongest acid contains only 
a single atom of water ; it is sold under the name of " glacial 
acetic acid," so called from its property of solidifying at a 
moderately low temperature. At about 50 deg. Fahr. the 
crystals melt, and form a limpid liquid of pungent odor and a 
density nearly corresponding to that of water ; the specific 
gravity of acetic acid, however, is no test of its real strength, 
which can only be estimated by analysis. 

* N. B. — The arrangement in this vocabulary is made with reference to the 
second constituent in compounds. Thus, acetic ether wiW be found under 
the head of " ether, acetic," nitrate of silver as " silver, nitrate of," iodide of 
potassium as " potassium, iodide of," etc. Acids will be found under their 
special names. 



ALBDMEN. 33 

The commercial glacial acetic acid, so termed, is usually 
diluted with water, and sometimes a trace of sulphurous acid 
is introduced, as the writer is informed, to confer the property 
of solidifying in cold weather, and thus to give an appearance 
of strength. It is, however, probably due in most cases to the 
decomposition of the sulphuric acid used to obtain it. Sul- 
phurous and hydrochloric acids are both injurious in photo- 
graphic processes, from their property of precipitating nitrate 
of silver. To detect them proceed as follows : — Dissolve a 
small crystal of nitrate of silver in a few drops of water, and 
add to it about half a drachm of the glacial acid ; the mixture 
should remain quite clear even when exposed to light. Hydro- 
chloric and sulphurous acid produce a white deposit of chloride 
or sulphite of silver, distinguishable by nitric acid, which 
dissolves the sulphite, but leaves the chloride unchanged ; and 
if aldehyde or volatile tarry matter be present in tlie acetic 
acid, the mixture with nitrate of silver, although clear at iirst, 
becomes discolored by the action of light. 

Glacial acetic acid sometimes has a smell of garlic. In this 
case it probably contains an organic sulphur acid, and is unfit 
for use. 

Many employ a cheaper form of acetic acid, sold by druggists 
as " Beaufoy's" acid ; it should be of the strength of the acetic 
acid fortiss. of the London Pharmacopoeia, containing 30 per 
cent, real acid. It will be advisable to test it for sulphuric 
acid, and other impurities, before use. When a certain quan- 
tity of the glacial acid is advised in a formula, take three 
times as much of the Beaufoy's acid. 

ALBUMEN. 

Albumen is an organic principle found both in the animal 
and vegetable kingdom. Its properties are best studied in 
the wfiite of egg, which is a very pure form of albumen. 

Albumen is capable of existing in two states ; in one of 
which it is soluble, in the other insoluble, in water. The 
aqueous solution of the soluble variety gives a slightly alkaline 
reaction to test-paper; it is somewhat thick and glutinous, but 
becomes more fluid on the addition of a small quantity of an 
alkali, such as potash or ammonia. 

Soluble albumen may be converted into the insoluhle form 
in the following ways : — 

1. By the Application of Heat. — A moderately strong solu- 
tion of alljumen becomes opalescent and coagulates on being 



34 OUTLINES OF GENERAL CHEMISTRY. 

heated to about 150 deg. Fahr., but a temperature of 212 deg. 
is required if the liquid is very dilute. A layer of dried al- 
bumen is not rendered insoluble by dry heat of 212 deg. 

2. By Addition of Strong Acids. — Nitric acid coagulates al- 
bumen perfectly without the aid of heat. Acetic acid, how- 
ever, acts differently, appearing to enter into combination 
with the albumen, and forming a compound soluble in warm 
water acidified by the acetic acid. 

3. By the Action of Metallic Salts. — Many of the salts of 
the metals coagulate albumen completely. Nitrate of silver 
does so ; also the bichloride of mercury. Ammonia-nitrate of 
silver, however, does not coagulate albumen. 

The white precipitate formed on mixing albumen with 
nitrate of silver is a chemical compound of the animal matter 
with protoxide of silver, and has been termed albuminate of 
silver ; its properties will be described afterwards. On heating 
in a current of hydrogen gas, it assumes a brick-red color, 
being probably reduced to the condition of a compound with 
suboxide of silver. It is then almost insoluble in ammonia, 
but enough dissolves to tinge the liquid wine-red. The red 
coloration of solution of nitrate of silver employed in sensi- 
tizing the albuminized photographic paper is doubtless pro- 
duced by the same compound, although of ten referred to the 
presence of sulphide of silver. 

Albumen also combines with lime and baryta. When chlor- 
ide of barium is used with albumen, a white precipitate of 
this kind usually forms. 

By long keeping, albumen loses its alkaline reaction and 
becomes sour and more limpid than at first. Mucous threads, 
like cobwebs, form in it, which appear to be caused by oxida.- 
tion. Ammonia added to albumen is said to preserve it 
for a longer time, and a lump of camphor floated in the 
liquid has also a good effect. Decomposed albumen usually 
contains sulphuretted hydrogen. 

Chemical Composition of Albumen. — The following is the 
composition of albumen according to Lieberkuhn : — 

Carbon ...... 53.3 

Hydrogen . . - . . . . 7.1 

Nitrogen , . . . . . 15.Y 

Oxygen . . . . . . 22.1 

Sulphur . . . . . . 1.8 

100.0 



ALCOHOL. 35 

ALCOHOL. 
Formula, C^H O. 

Alcohol is obtained bj the careful distillation of any spirit- 
uous or fermented liquor. If wine or beer be placed in a re- 
tort, and heat applied, the alcohol, being more volatile than 
water, rises first, and may be condensed in an appropriate re- 
ceiver; a portion of the vapor of water, however, passes 
over with the alcohol, and dilutes it to a certain extent, for- 
ming what is termed " spirit of wine." Much of this water 
may be removed by redistillation from carbonate of potash ; 
but in order to render the alcohol thoroughly anhydrous, 
it is necessary to employ quicklime, which possesses a still 
greater attraction for water. For this purpose strong alcohol of 
.823 should be left in contact with powdered quicklime for three 
or four days, or until the latter has ceased to swell from ab- 
sorption of water, after which it is separated by distillation, 
the retort being placed in a water-bath. 

Properties of Alcohol. — Pure anhydrous or absolute alcohol 
is a limpid liquid, of an agreeable odor and pungent taste ; 
sp. gr. at 60 deg., .794. It absorbs vapor of water, and becomes 
diluted by exposure to damp air ; boils at 173 deg. Fahr. It has 
never been frozen. 

Alcoliol distilled from carbonate of potash has a specific 
gravity of .815 to .823, and contains 90 to 93 per cent, of real 
spirit. 

The specific gravity of ordinary rectified spirits of wine is 
about .836, and it contains 80 to 83 per cent, of absolute 
alcohol. 

Different Commercial Qualities of Alcohol. — Alcohol really 
absolute is not often used in photography ; the expense of 
making it is very great, and it could not be prepared at a pro- 
fit. A spirit with less than four per cent, of water (sp. gr. 
.805) may be obtained by agitating commercial spirit of wine 
first with carbonate of potash in the manner presently to be 
advised, and then with a common quality of dry chloride of 
calcium. Put in about three-quarters of a pound of the chlor- 
ide of calcium to half a gallon of spirit of .815 ; the greater 
part dissolves with perceptible rise of temperature. Draw 
over as much as possible in a steam bath ; and to prevent the 
residue in the retort from setting into a hard mass, it is well to 
add a little water to it, after the distillation is completed. In 
this way the commercial absolute alcohol is usually prepared. 



36 OUTLINES OF GENERAL CHEMISTRY. 

The next quality of spirit is the strong alcohol of .815 to 
.823. This may be obtained by agitating spirit of wine, .836, 
with an excess of dry carbonate of potash. The salt termed 
carbonate of potash is a deliquescent salt, having a great attrac- 
tion for water ; consequently, when spirit of wine is shaken 
with carbonate of potash, a portion of water is removed, the 
salt dissolving in it and forming a dense liquid, which refuses 
to mix with the alcohol, and sinks to the bottom. At the ex- 
piration of two or three days, if the bottle has been shaken 
frequently, the action is complete, and the lower stratum of 
fluid may be drawn off and rejected. 

In order to obtain the greatest amount of concentration, it 
will be necessary to have a series of vessels, each containing . 
about a pound of the carbonate to a gallon of spirit. The al- 
cohol may be passed from one to the other, and should not be 
distilled until the finely-powdered carbonate can be shaken 
about in the liquid without being wetted. The carbonate 
which the writer employs is nearly pure, and costs from a 
shilling to eighteen pence per pound ; it must be dried in a 
hot metal plate before use. A commoner variety would an- 
swer, but it lias the disadvantage of clotting together at the 
bottom of the vessel, and of not dissolving into a clear liquid. 

A third quality of commercial alcohol is the rectified spirit 
of wine of .836 already referred to ; it is very suitable for 
adding to developing fluids, etc., but not sutticiently strong 
for good collodion. 

Alcohol for Collodion Photography. — For a long time the 
writer was in the habit of employing rectified spirit of wine 
for the preparation of collodion, increasing its strength as far 
as necessary, by means of dry carbonate of potash ; but having 
at length become dissatisfied with the smell of certain samples 
of this rectified spirit, he has since used a strong alcohol ob- 
tained by one distillation. In rectifying spirit, a liquid known 
as " faints " is sometimes mixed with the purer spirit for the 
sake of economy, and these faints are invariably contaminated 
with essential oils. It is most important in photography to 
avoid essential oils, and therefore the grain spirit obtained by- 
one distillation in a Coffey's still may be preferred. In 
taking tlie specific gravity of this grain of spirit, which varies 
from. 817 to. 819 at 60 deg, Fahr., we see at once the advantage 
Ekely to accrue from its employment, since the fusel oil, 
wliich boils at a more elevated temperature, cannot rise so 
high in the still, and is separated. The smell of the grain 
spirit is very sweet ; and although it is not quite so strong as 



ALUM. 37 

is required, yet by converting a portion of it into alcoliol of 
.805, by means of dry chloride of calcium, and mixing this 
with the remainder, the correct specific gravity may easily be 
obtained. 

The reaction to test-paper of the pure grain spirit should 
be quite neutral ; but in some instances a trace of acid is pre- 
sent, so that each half-gallon of spirit requires about one drop 
of solution of ammonia of sp. gr. 0.93. The writer has never 
yet found in this or in any other spirit the alkaline reaction 
which is exhibited by ether. 

ALCOHOL {Methylated). 

Spirit of wine containing ten per cent, of wood naphtha 
is allowed by the excise law to be sold free of duty, under 
the name of " methylated spirit." The quality, however, is 
often inferior, since residues containing fusel oil and other 
volatile bodies are usually rectified for the purpose of methy- 
lating. 

ALCOHOL, METHYLIC ( Wood JSTaphtha— 
Pyroxylic Spirit). 

Formula, CH,0. 

This liquid is one of the products of the destructive dis- 
tillation of wood, and constitutes a large portion of the in- 
flammable fluid called wood naphtha, or pyroxylic spirit. In 
its pure state it much resembles common alcohol in its pro- 
perties. As common alcohol by oxidation furnishes an acid, 
the acetic, so mythylic alcohol in like manner yields a simi- 
lar acid, called formic acid. 

ALUM. 

Formula, AlK(SOi)2+12H20 = 474.5. 

The common alum of commerce, or potash alum, is found 
native in some places, but it also manufactured on an exten- 
sive scale. It is soluble in 18 parts of cold water and in 
rather less than its own weight of boiling water. It tastes 
astringent and acidulous, is styptic, reddens litinus, and loses 
its water of crystallization by being heated. 

Its use in photography is mainly confined to imparting 
hardness and insolubility to gelatine films on either glass or 



38 OUTLINES OK GENERAL CHEMISTRY. 

paper, for which purpose chrome alum is also employed. 
When mixed with citric or other similar acids it serves a 
useful purpose in clearing gelatine negatives that have be- 
come discolored by the developer. 

Much of the common alum contains ammonia as well as 
potassium. 

AMMONIA. 

Formula, ^113= 17. 

The liquid known by this name is an aqueous solution of 
the volatile gas ammonia. Ammoniacal gas contains one atom 
of nitrogen combined with three of hydrogen ; these two ele- 
mentary bodies when free, show no tendency to combine, 
but they can be made to unite under certain circumstances, 
and the result is ammonia. 

Properties of Ammonia. — Ammoniacal gas is soluble in 
water to a large extent ; the solution possessing those proper- 
ties which are termed alkaline. Ammonia, however, differs 
from the other alkalies in one important particular — it is 
volatile ; hence, the original color of turmeric-paper affected 
by ammonia is restored on the application of heat. Solution 
of ammonia absorbs carbonic acid rapidly from the air, and 
is converted into carbonate of ammonia ; it should therefore 
be preserved in stoppered bottles. Besides carbonate, com- 
mercial ammonia often contains chloride of ammonium, re- 
cognized by the white precipitate given by nitrate of silver 
after acidifying with pure nitric acid. 

The strength of commercial ammonia varies greatly ; that 
sold for pharmaceutical purposes under the name of liquor 
ammonise contains about ten per cent, of real ammonia. The 
sp. gr. of aqueous ammonia diminishes with the portion of 
ammonia present, the liquor ammonise being usually about 
.936. It is a common error in photographic operations to 
confound the diluted ammonia with the liquor ammonise fort., 
which is so strong that a single drop will neutralize a large 
portion of acid, and has sp. gr. of only .88. 

Ammonia has no resemblance in composition to other bases 
which are metallic oxides, and it differs from them also, in 
that it never combines with oxygen acids without including 
water as an essential ingredient in the compound. Yet the 
salts of ammonia have a close resemblance in properties to 
those of the alkalies potash and soda, and this resemblance 
has led to the ammonium, theory, which develops a close re- 



AMMONIA. 39 

semblance in composition between the salts of ammonia and 
of the alkalies, and also explains why water is essential to the 
salts with oxygen acids. 

AMMONIA, BICHROMATE OF. 

Formula, NII,Cr20, = 234.4. 

This salt is obtained by dividing a solution of chromic acid 
into two parts, neutralizing one with ammonia, then adding 
the other part and evaporating. The resulting crystals are 
permanent and soluble in water. 

Bichromate of ammonia is used in many photographic pro- 
cesses, dependent on the peculiar property of this salt in ren- 
dering certain organic bodies, such as albumen, gelatine, etc., 
insoluble in their usual menstrua, after exposure to light. 

The rationale of the various processes seems to be this. 
The bichromate, in contact with the gelatine or other organic 
body and exposed to light, gives up part of its oxygen to the 
organic body, which is thus rendered insoluble in warm water 
in exact proportion to the extent of the actinic action. 

AMMONIA, CARBONATE OF. 

This occurs commercially in lumps of a considerable size, 
obtained by sublimation. Its composition is rather variable. 
"When first formed it has nearly the composition of a sesqui- 
carbonate, but by exposure to the air neutral carbonate of am- 
monia escapes, and a white powder is left, which is a bicar- 
bonate. 

AMMONIUM SULPHIDE (or Sulphydrate). 
Formula, NH„HS = 51. 

This solution is formed by passing sulphuretted hydrogen 
through ammonia. It is used in photography for precipita- 
ting silver solutions, and for intensifying negatives. The lat- 
ter object it accomplishes by changing the deposit on the de- 
veloped image into a dense black color which obstructs the 
actinic rays. 

The solution should be kept in a well -stoppered bottle, and at 
a distance from the dark room, because the fumes given off are 
exceedingly prejudicial to other operations which have to be 
performed there. 



40 OUTLINES OF GENERAL CHEMISTRY. 

AMMONIA, NITRATE OF. 

Formula, NH^NOg^-SO. 

This is a neutral combination of nitric acid and ammonia, 
which may be crystallized without difficulty. It is gradually 
formed in the photographic nitrate bath when compounds of 
ammonium are used in iodizing. 

Nitrate of ammonia is not of itself alkaline, but inasmuch 
as it is a solvent of oxide and also of carbonate of silver, a bath 
containing nitrate of ammonia will give a strong alkaline re- 
action on adding to it either potash, ammonia or chalk. 

"When heated, nitrous oxide, or laughing gas, is evolved. 
Its solution in water is attended by an intense degree of cold. 

AMMONIUM, BROMIDE OF. 
Formula-, NH:,Br = 98. 

This is a crystallized salt, which may be prepared by pre- 
cipitating bromide of calcium by carbonate of ammonia. It is 
very soluble in water, and is more easily dissolved by alcohol 
and ether than the corresponding bromide of potassium. It 
does not become colored on kee]3ing, like the iodide of am- 
monium. 

Bromide of ammonium is a better form of bromide for col- 
lodion than the corresponding salt of potassium or cadmium, 
and it can usually be obtained in a pure state. 

AMMONIUM, CHLORIDE OF. 

Formula, NH,C1= 53.5. 

This salt, also known as muriate or hydrochlorate of am- 
monia, occurs in commerce in the form of colorless and trans- 
lucent masses, which are procured by sublimation^ the dry 
salt being volatile when strongly heated. It dissolves in an 
equal weight of boiling, or in three parts of cold water. It 
contains more chlorine in proportion to the weight used than 
chloride of sodium, the atomic weights of the two being as 
53.5 to 58.5. 

Chloride of ammonium is easily obtained in a pure state, 
and is, on the whole, more suitable for salting paper than 
either chloride of sodium or chloride of barium. 



AESENIC. 41 

AMMONIUM, IODIDE OF, 

Formula, NHJ = 145, 

Is a salt very valuable in collodion, because it has the property 
of conferring limpidity, sensitiveness and adherency to the 
glass, which other iodides do not always possess. It is, how- 
ever, an unstable substance, prone to liberate iodine and to de- 
' compose the collodion in which it is dissolved, unless this bad 
effect is neutralized by iodide of cadmium, which has an op- 
posite tendency. 

To decolorize iodide of ammonium which has been decom- 
posed by keeping, shake it up with a little strong ether ; the 
iodine will be dissolved out, and unless alcohol be present, no 
great loss from solution will result. 

AMMOmUM, SULPHOCYANIDE OF. « 

[See SULPHOCYANIDES.) 

ANILINE. 

Formula, CgH^N = 93. 

This powerful base is derived from indigo, nitrobenzol, 
coal-tar, etc. 

When pure, aniline is a thin, colorless, and highly refract- 
ing oil, of a burning taste and aromatic flavor. "With acids it 
forms a remarkable series of salts, which crystallize with great 
beauty and facility. 

With chromic acid it gives a deep green or bluish-black 
color, which has been taken advantage of by Mr. Willis in his 
photographic process for copying drawings. 

^,^ AQUA REGIA. {See Nitro-Hydeochlokic Acid.) 

ARSENIC, BROMIDE OF. 

Formula, AsBr3 = 315. 

Bromide of arsenic has been used in collodion for the pur- 
pose of increasing the intensity of the developed image. A 
solution suitable for that purpose may be prepared by reducing 
metallic arsenic to a fine powder and placing it in a dry bottle 
with alcohol of sp. gr. .805, Bromine is then to be dropped 



42 OUTLINES OF GENERAL CHEMISTEY. 

into the alcohol, when immediate combination will ensue ; the 
arsenic must always be in excess. 

The addition of water decomposes bromide of arsenic into 
areenious acid and hydrobromic acid ; hence the necessity for 
using alcohol of considerable strength. 

AUKO-CHLORIDE OF SODIUM. 

{See Sodium, Auro-Chloride of.) 

BARIUM, CHLORIDE OF. 

Formnla, BaCl^ + 2 Aq. = 244. 

Barium is a metallic element very closely allied to calcium, 
the elementary basis of lime. The chloride of barium is com- 
« monly employed as a test for sulphuric acid, with which it 
forms an insoluble precipitate of sulphate of baryta. It also 
slightly alters the color of the photographic image when used 
in preparing positive paper, which may be due, in some measure, 
to a chemical combination of baryta with albumen ; but it 
must be remembered that this chloride, from its high atomic 
weight, contains less chlorine than the alkaline chlorides. 

Properties of Chloride of JBarium. — Chloride of barium 
occurs in the form of white crystals, soluble in about two parts 
of water, at common temperatures. These crystals contain 
two atoms of water of crystallization, which are expelled at 
212 deg., leaving the anhydrous chloride. 

BARYTA, NITRATE OF. 

Formula, Ba(E'03)2 = 261. 

Nitrate of barium forms octohedral crystals, which are anhy- 
drous. It is considerably less soluble than the chloride of 
barium, requiring for solution twelve parts of cold and four of 
boiling water. It may be substituted for the nitrate of lead 
in the preparation of protonitrate of iron. Its addition to the 
negative nitrate bath prevents the formation of pin-holes in 
the collodin film 

BENZOLE. 

Formula, CisHs^TS. 

A limpid liquid, obtained commercially by distilling off the 
most volatile constituant of the substance known as " coal 



BKOMINE, 43 

naphtha." It does not mix with water, but is dissolved in any 
quantity by alcohol or ether. 

Benzole is an excellent solvent of fats and oils, and may be 
employed for removing grease-spots. It also dissolves gutta- 
percha and caoutchouc. A rapidly drying varnish may be 
made with benzole, but it should iirst be purified by redis- 
tillation, since ordinary benzole sometimes leaves a greasy 
residue on drying. 

BITUMEN OF JUD^A, OR ASPHALTUM. 

This is an indurated pitch found in the Dead Sea, in Trini- 
dad, and many other places. 

It is the basis of most black varnishes, being soluble in 
naphtha and some other substances. 

Being sensitive to light, it has also been used in several 
photographic and photo-engraving processes. 

BROMIDES, TEST FOR PURITY. 

The most common impurity in bromides is an iodide. This 
impurity is of little or no consequence in the wet collodion 
process, or when sensitizing bromized collodion in a nitrate 
bath ; but it is prejudicial in all the emulsion processes, because 
the particles of iodide of silver thus formed are much coarser 
than the bromide, falling to the bottom of the collodion very 
speedily and carrying with them a proportion of bromide. 

Dissolve four or five grains of the suspected bromide in an 
ounce or two of distilled water in a test-tube. In another 
similar test-tube have distilled water only. Now dip the end 
of a clean glass rod into a solution of chloride of jialladium, 
and with it stir up the bromide solution. With another rod 
do the same with the distilled water. If iodide is absent, both 
tubes, when held up to the light, will be equally transparent ; 
but if the least trace of iodide be mixed with the bromide a 
brownish red color will instantly be struck. 

This is a most delicate test for an iodide, and should never 
be omitted when one wishes to have his bromized emulsion of 
silver in the finest possible state. 

BROMINE. 

Symbol, Br =80. 

This elementary substance is obtained from the uncrystal- 
lizable residium of sea-water, termed iittern It exists in the 



44 OUTLINES OF GENERAL CHEMISTRY. 

water in very minute proportions, combined with magnesium 
in the form of a soluble bromide of magnesium. 

Properties. — Bromine is a deep reddish brown liquid of a 
disagreeable odor, which gives off ruddy vapors at common 
temperatures ; sparingly soluble in water (1 part in 23, Lowig), 
but more abundantly so in alcohol, and especially in ether. It 
is very heavy, having a specific gravity of 3.0. 

Bromine is closely analogous to chlorine and iodine in its 
chemical properties. It stands intermediately between the 
two; its affinities being stronger than those of iodine, ^but 
weaker than chlorine. 

It forms a large class of salts, of which the bromides of po- 
tassium, ammonium, cadmium and silver are the most familiar 
to photographers. 

CADMIUM. 

Symbol, Cd= 56. 

A white metal, resembling tin in its physical properties. 
Its oxide is found associated with zinc in certain of the ores of 
the latter, but the two are easily separated. 

The iodide and bromide of cadmium are used in photo^ 
raphy, on account of their permanency, and the facility wit 
which they dissolve in ether and alcohol. 

Metallic cadmium is sometimes employed to remove free 
iodine from collodion. 

CADMIUM, BROMIDE OF. 
Formula, Cd,Br2 + ^H^O = 344. 

This salt is much used in photography, on account of its 
great stability, and its solubility in collodion. 

It is prepared by heating an excess of the filings of the metal 
cadmium with hydrobromic acid and water. By evaporation 
acicular crystals are formed, which constitute the bromide of 
commerce, and contain four equivalents of water of crystalli- 
zation. This water is expelled by heating the bromide in a 
porcelain evaporating dish on a sand-bath placed over a Bun- 
sen's gas-burner. After a time the bromide melts into a pasty 
mass in its own water of crystallization, and at last solidifies 
into a hard cake. It is better, however, when the bromide is 
in the pasty state to keep stirring it occasionally with a glass 
rod, to promote the complete expulsion of the water. The 
product is now pounded up and kept in a stock bottle. 



CALCIUM. 4:5 

If the heat is raised too high a risk is run of the bromide 
decomposing, the symptoms of which is the liberation of bro- 
mine, most easily detected by the sense of smell. In such case 
remove the flame or the dish for a short time until the tem- 
perature has been brought lower. 

CADMIUM, IODIDE OF. 
Formula, Cdl2 = 366. 

QThis salt is formed by heating filings of metallic cadmium 
with iodine, or by mixing the two with addition of water. 

Iodide of cadmium is very soluble, both in alcohol and water ; 
the solution yielding on evaporation large six-sided tables of 
a pearly lustre, which are permanent in the air. The com- 
mercial iodide is sometimes contaminated with iodide of zinc, 
the crystals being imperfectly formed, and slowly liberating 
iodine when dissolved in ether and alcohol. Pure iodide of 
cadmium remains nearly or quite colorless in collodion, if the 
fluid be kept in a cool and dark place. 

CALCIUM, BROMIDE OF. 

Formula, CaBr^ + 4H3O = 272. 

This salt may be obtained in various ways. The simplest, 
and perhaps the best, plan is by saturating hydrobromic acid 
with carbonate of lime. 

Its aqueous solution, when evaporated, yields silky hydrated 
crystals. It is not much used in photography. 

CALCIUM, CHLORIDE OF. 

Formula, CaCl2 = 111. 

This salt is found in sea-water ; but is usually prepared arti- 
ficially by dissolving chalk in hydrochloric acid and evaporat- 
ing. When strongly dried it occurs in lumps which are hard 
and difficult to pulverize. 

Chloride of calcium has a great attraction for water, and is 
used for drying gases and other purposes. Exposed to the air, 
it soon deliquesces from absorption of atmospheric moisture. 
It is very soluble in alcohol with evolution of heat, and when 
the liquid is subjected to distillation, a highly concentrated 
spirit passes over, leaving a viscid mass which crystallizes on 



46 OTTTLINES OF GENERAL CHEMISTRY. 

cooling, and contains nearly sixty per cent, of alcohol in a state 
of loose chemical combination with the chloride. 

Pure dry chloride of calcium is an expensive salt, but a com- 
mon quality, sufficiently good for the use above described, 
may be obtained at a lower price. 

CALCIUM, IODIDE OF. 

Formula, Cal^ = 294. 

This salt, useful for the preparation of some other iodides, 
may be obtained either by saturating hydriodic acid with car- 
bonate of lime, or by digesting iodine with metallic iron and 
water until the liquid is purely green, then adding excess of 
lime and filtering off the solution from the precipitate and 
evaporating. 

CAMPHOK. 

Formula, C2oHi60,= 152. 

This substance is obtained by distillation from the camphor 
laurel of China and Japan. When a small piece is added to 
solutions of gelatine, albumen, tannin, etc., it tends to pre- 
vent them from decomposition and mould. 

CANADA BALSAM. 

Is a kind of inspissated turpentine obtained from a species 
of fir tree. In photography it is used for cementing the com- 
ponents of achromatic lenses, and when dissolved in benzole 
for rendering paper highly translucent. 

CAOUTCHOUC. 

This substance, known also as India rubber, is the inspissated 
milky juice of trees growing in South America and the East 
Indies. It is insoluble in water and in alcohol. 

Chloroform is the most perfect solvent for caoutchouc, and 
leaves it unchanged on evaporation. Benzole also acts upon it. 
Mineral naphtha takes it up on applying heat, but the residue 
after evaporation is sticky. 

By combination with sulphur, caoutchouc undergoes a 
change of properties, familiar tons in the article sold as vul- 
canized India-i'uhher. 



CHLOKINE. 47 

CARBOLIC ACID. 

Formula, CelleO = 94. 

Like creasote, this substance is derived from wood or coal 
tar. It is a powerful antiseptic, or preventive of putrefac- 
tion, more so tlian creasote. For this purpose a very minute 
quantitv added to albumen, etc., w^ill prevent decay or mould, 

CASTOR OIL. 

This oil is extracted from the seeds of Iticinus communis, or 
Palma Christi, both of which are cultivated in warm climates. 

In photography it is used in a small quantity to confer 
toughness on collodion for transferring from the glass, and 
also to give the same property to photographic varnishes. 

CHAECOAL, ANIMAL. 

Animal charcoal is obtained by heating animal substances, 
such as bones, dried blood, horns, etc., to redness, in close ves- 
sels, until all volatile empyreumatic matters have been driven 
oif, and a residue of carbon remains. When prepared from 
bones, it contains a large quantity of inorganic matter in the 
shape of carbonate and phosphate of lime. Animal charcoal is 
freed from these earthy salts by repeated digestion in hydro- 
chloric acid ; but unless very carefully washed it is apt to re- 
tain an acid reaction, and so to liberate free nitric acid when 
added to solution of nitrate of silver. 

Properties. — Animal charcoal, when pure, consists solely of 
carbon, and burns away in the air without leaving any residue; 
it is remarkable for its property of decolorizing solutions; the 
organic coloring substance being separated, but not actually 
destroyed, as it is by chlorine employed as a bleaching agent. 
This power of absorbing coloring matter is not possessed in 
an equal degree by all varieties of charcoal, but is in great 
measure peculiar to those derived from the animal kingdom. 

CHLORINE. 

Symbol, CI =35.5. 

Chlorine is an element, found abundantly in nature, com- 
bined with metallic sodium in the form of chloride of sodium, 
or sea-salt. 



48 OUTLINES OF GENBEAL CHEMISTRY. 

Preparation. — By tlie action of liydrochloric acid on a nat- 
ural product known as binoxide of manganese, MnOg. The 
reaction may be thus represented : — 

MnO^+^HCl = MnCl^ + II.O + 2C1. 

Properties. — Chlorine is a greenish-yellow gas of a pungent 
and suffocating odor ; soluble to a considerable extent in 
water, the solution possessing the odor and color of the gas. 
It is nearly two and a half times as heavy as a corresponding 
bulk of atmospheric air. 

Chemical Properties. — Chlorine belongs to a small natural 
group of elements which contain also bromine, iodine, and 
iluorine. They were characterized by having a strong afhnity 
for hydrogen, and also for the metals ; but are comparatively 
indifferent to oxygen. Many metallic substances actually un- 
dergo Gombustion when projected into an atmosphere of chlor- 
ine, the union between the tw^o taking place with extreme 
violence. The characteristic bleaching properties of chlorine 
gas are explained in the same manner : — Hydrogen is removed 
from the organic substance, and in that way the structure is 
broken up and the color destroyed. 

Chlorine is more powerful in its affinities than either brom- 
ine or iodine. The salts formed by these three elements are 
closely analogous in composition, and often in properties. 
Those, of the alkalies, alkaline earths, and many of the metals, 
are soluble in water ; but the silver salts are insoluble ; the 
lead salts sparingly so. 

The combinations of chlorine, bromine, iodine, and fluorine, 
with hydrogen, are acids, which by combination with bases, 
form haloid salts and water. 

The test by which the presence of chlorine is detected, in its 
free state, or in its soluble metallic compounds, is nitrate of 
silver ; it gives a white curdy precipitate of chloride of silver, 
insoluble in nitric acid, but soluble in ammonia. The solution 
of nitrate of silver employed as the test must not contain 
iodide of silver, since this compound is precipitated by 
dilution. 

CHLOEOFOEM. 

Formula, CJICls^ 119.5. 

This volatile liquid is obtained by the action of chloride of 
lime upon dilute alcohol. It does not mix with water, but is 
very soluble in spirit. 



CITRIC ACID. 49 

Chloroform is the best solvent known for caoutchouc, and 
it also dissolves gutta-percha readily. Amber and many other 
resins are more or less soluble in chloroform ; and this solvent 
is well suited for the preparation of photographic varnishes, 
from its volatility, and from its having no solvent power on 
any of the varieties of collodion films, some of which would 
be dissolved by an alcohol varnish. 

CHROMIUM. 

Symbol, Cr= 26.3. 

Chromium is a metallic element found in the state of oxide 
combined with oxide of iron in the mineral termed chrome 
ironstone. Its most important oxides are the basic oxide, 
CrgOg and chromic acid, CrOs. 

Chromate of potash, the source of all the preparations of 
chromium, is obtained from the ore by fusing it for a long 
time with nitrate of potash ; the latter imparts oxygen, which 
changes CrgOg into two atoms of CrOs, which with the potash 
forms chromate; this is afterwards dissolved out and crystal- 
lized. 

The chromate acidified with nitric acid yields the bichrom- 
ate of potash, which is purified by crystallization. 

citric acid. 

ii3,c;ha+H20=2io. 

This acid is derived from the juice of lemons and other 
fruits. It is made to undergo a short fermentation in order 
that impurities may subside. It is then neutralized with chalk, 
by which citrate of calcium, an insoluble compound, is 
formed. After washing, this citrate is decomposed with dilute 
sulphuric acid ; the sohition is evaporated and left to crystallize. 
The product maybe purified by another process ; but for pho- 
tographic work this is not necessary. 

Citric acid is a powerful retardent to the action of the de- 
veloper, much more so than acetic acid. Hence, in hot 
weather, it is much used ; and, for the same reason, it is 
also very useful wdien intensifying a negative, as it tends to 
keep the shadows clear. 

A small proportion of citric acid may be added with great 
advantage to the nitrate solution for sensitizing positive paper, 



50 OUTLINES OF GENERAL CHEMISTRY. 

when there is too much tendency exhibited by the paper to 
print of a pale slaty tone. 

CYANOGEN. 

Formula, CN=26. Symbol, Cy. 

Cyanogen is a very remarkable compound, which behaves 
in many respects like the elements, chlorine, bromine and 
iodine. Like them, it unites with hydrogen to form an acid, 
hydrocyanic or prussic acid ; and, like them, it combines with 
metals to form haliod salts. Cyanide of silver, AgCN or 
AgCy, much resembles chloride of silver in properties. 

Cyanogen has a capacity for forming complex compounds 
with certain metals, in which the metals can no longer be 
discovered by the usual tests ; thus, iron for example enters 
into the composition of the ferrocyanides, but no ordinary 
test would discover it. 

C3^anogen in the uncombined state is a gas ; it cannot be 
formed directly from its elements, carbon and nitrogen, but 
may easily be procured by heating one of its compounds, 
cyanide of mercury, IIgCy2=Hg + 2Cy. 

ETHEE, SULPHUKIC. 

Formula, CJlwO=14:. 

It will be altogether unnecessary to describe the manufacture 
of this compound, because it can only be prepared properly by 
those thoroughly conversant with the process on a large scale. 
Sufficient be it to point out the means of ascertaining when 
one has got this most important photographic material in a 
state fit for his purpose. 

The first test is that of smell. If the ether smells of methyl 
or wood spirit, reject it without going farther ; but if it does 
not, proceed to test it farther with tincture of iodine. Put one 
drop of the latter into, say, an ounce of the ether. If the re- 
sulting color is discharged after a few hours, that ether is not 
to be trusted as it certainly contains too much methyl. The 
other test is for specific gravity, which should range from 720 
to not more than 730. 

When pure ether is exposed to light it ozonizes or becomes 
acid, and is liable to decompose the iodides, etc., with which 
it has afterwards to come in contact. Hence, it should always 
be kept in a dark and, for another reason, in a cool place. 



GALLIC ACID, 51 

Water is detected in ether by the turbidity which it causes 
when the latter is dropped into spirits of turpentine ; if no 
water is present, the two mix perfectly. 

An ozonized condition of ether is at once detected on 
agitating it with a little of a solution of iodide of potassium in 
water or alcohol, by its coloring the latter more or less yellow 
from the liberation of iodine. The difference between bad and 
good ether is seen most evidently after long keeping. Sup- 
posing white light to be excluded, a pure sample of ether may 
be placed in a bottle, only half full, and at the expiration of 
two or three months it will scarcely become colored on the 
first addition of iodide of potassium. Ether only partially 
purified will often stand the test of iodide of potassium when 
freshly distilled, but it will soon acquire theproperty of liberat- 
ing iodine when it is stowed away for keeping. 

ETHER, METHYLATED. 

This preparation is largely, indeed almost universally, used 
in the manufacture of the collodions of the present day. 
When properly rectified and purified it answers very well, 
and, being so much cheaper than the ethylic ether, has almost 
superseded that produced from pure alcohol. Its only bad 
tendencies are to disarrange the nitrate bath, and by its acrid 
fumes to affect the eyes of the operator when coating his 
glasses with collodion. 

ETHER, METHYLIC. 

Formula, C,IW-=4:Q. 

This ether bears the same relation to pyroxylic alcohol or 
wood spirit, that ordinary ether bears to alcohol from wine. 
It is a gaseous substance, which dissolves to a certain extent in 
water, but more abundantly so in alcohol or ether. 

GALLIC ACID. 

Formula, H,C,H5O5=lY0. 

Gallic acid is obtained from the tannic acid by a species of 
fermentation. The powdered galls mixed with water, or 
their infusion, is left for some weeks, during which it becomes 
mouldy. Oxygen is absorbed and carbonic acid is given off. 



62 OUTLINES OF GENERAL CHEMISTRY, 

and gallic acid is deposited in abundance. Tlie mouldy paste 
is squeezed, to get rid of foreign matters, and tlien boiled in 
water, wliicli on cooling, deposits the gallic acid. By digest- 
tion with animal charcoal, and recrystallization, it is obtained 
pure. » 

Gallic acid, like tannic acid, gives an intensely blue black 
color with salts of peroxide of iron, but it differs from the 
latter in giving no precipitate with gelatine. 

GELATINE. 

This is a nitrogenized organic substance somewhat'analogous 
to albumen, but differing from it in properties. It is obtained 
by subjecting bones, hoofs, horns, calves' feet, etc., to the 
action of boiling water. The jelly formed on cooling is termed 
size, or, when dried and cut into slices, glue. Gelatine, as it is 
sold in the shops, is a pure form of glue. Isinglass is gelatine 
prepared, chiefly in Russia, from the air bladders of certain 
species of sturgeon. 

Properties of Gelatine. — Gelatime softens and swells up in 
cold water, but does not dissolve until heated; the hot solution, 
on cooling, forms a tremulous jelly. One ounce of cold water 
will retain about three grains of isinglass without gelatinizing; 
but much depends upon the temperature, a few degrees greatly 
affecting the result. The solution forms an insoluble pre- 
cipitate with tannic acid, which has the composition of leather. 

When long boiled in water, and especially in presence of 
an acid, such as the sulphuric, gelatine undergoes a peculiar 
modification, and the solution loses either partially or entirely 
its property of solidifying to a jelly. 

GLYCEKINE, 

Formula, C3H803=92. 

Fatty bodies are resolved by treatment with an alkali into 
an acid, which combines with the alkali, forming a soap and 
glycerine, which remains in solution. 

Pure glycerine, as obtained by Price's patent process of dis- 
tillation, is a sweet viscid liquid of sp. gr. about 1.23 ; miscible 
in all proportions with water and alcohol. It is neutral to test 
])aper. It has little or no action upon nitrate of silver in the 
dark, and reduces it very slowly even when exposed to light. 
Glycerine has been used as a preservative for keeping wet 
collodion films from drying during long exposures. 



GOLD. 53 

GOLD, CHLORIDE OF. 

Formula, AuCl3=303.5. 

There are two clilorides of gold, viz., the protochloride and 
the terchloride. The latter is the one used in photography. 
It is prepared bj dissolving gold in aqua regia or nitro-hydro- 
chloric acid. 

Absolutely pure chloride of gold can only be made from the 
pure metal, but as purity is not an essential for photographic 
purposes, the following method of making it from standard 
gold coin, which also applies to the pure metal, will be found 
convenient. The Australian gold coins, in which the alloy is 
silver, are preferable to those of the English mint, inasmuch 
as the silver is left in the shape of undissolved chloride, which 
can be afterwards filtered out, whereas copper is much more 
difficult to get rid of. The mode of procedure is as follows : 

Mix in a rather tall and thin German beaker two fluid 
drams of nitric with one ounce of hydrochloric acid. Unless 
the acids are very strong, the addition of water will be unnec- 
essary. Place in the mixture, say, an Australian sovereign, 
and apply a gentle heat, which should be continued until solu- 
tion is complete. A water-bath or a warm hob will give suffi- 
cient heat to commence and continue the action. In most 
cases, the above pro]3ortion of acids will be sufficient to dis- 
solve the sovereign, if the heat is continued ; but if not, add a 
little more mixed acid. A great excess of acid should be 
avoided, because it renders their neutralization, or their subse- 
quent elimination, more difficult. 

When solution of the gold is complete, there will remain a 
precipitate of chloride of silver, arising from the alloy. Dilute 
down, say, with six ounces of distilled water, neutralize the 
excess of acids with powdered chalk, and filter into a stock 
bottle. But in order to avoid the least trace of waste, rinse 
out the beaker with two ounces more of distilled water, which 
pass through the filter into the stock bottle. Repeat the 
rinsing, etc. Thus we have the whole, or nearly the whole, 
of tJie gold in a sovereign converted into chloride of gold fit 
for photographic purposes. It is avisable to keep this neutral 
solution of chloride in the dark, because light has the eifect of 
causing a partial reduction of the metallic element. 

As a sovereign contains 113 grains of pure gold, it is easy to 
calculate that, if waste is avoided, it will yield 174 grains of 
chloride. 



54 OUTLINES OF GENERAL CHEMISTRY. 

Supposing, then, the chloride of gold thus made from a 
sovereign is diluted, say, with ten ounces of distilled water, 
each fluid ounce will contain seventeen grains of the chloride, 
leaving four grains to be laid to the account of waste, or a 
light sovereign. 

GOLD, HYPOSULPHITE OF. 

Formula, Au2SgO3=506. 

This salt, which is produced by the reaction of chloride of 
gold on hyposulphite of soda, is now rarely employed in pho- 
tography. In former times, it was much used for toning 
Daguerreotype images, and went under the name of Sel 
6) Or. 

Being a very unstable substance, it is totally unfitted for 
toning photographic paper prints, for which purpose it has 
been sometimes recommended. 

GUMS. 

These may be shortly described as exudations from various 
kinds of trees. They are all more or less soluble in water, 
and this distinguishes them from "resins," which are insoluble 
in that menstruum. 

In photography, some of them are used as pastes for mount- 
ing photographs, and dilute in solution as preservatives or 
organifiers in the dry collodion processes. The only drawback 
to their employment for the latter purpose is their tendency, 
when conjoined with collodion, to cause blistering of the film 
during or after, development. 

Gum arable may be considered the best type of that class of 
substances called •' gums." 

HYDROBROMIC ACID. 

Formula, HBr=Sl. 

This is prepared by decomposing bromide of potassium with 
a concentrated solution of phosphoric acid, or by decomposing 
bromide of phosphorous by means of a small quantity of water. 

HYDRIODIC ACID. 

Formula, HI=128. 

This is a gaseous compound of hydrogen and iodine, corres- 
ponding in composition to the hydrochloric acid. It cannot, 



hydkogf:n, 55 

however, from its instability, be obtained in the same manner, 
since, on distilling an iodide with sulphuric acid, the hydriodic 
acid first formed is subsequently decomposed by the sulphuric 
acid. An aqueous solution of hydriodic acid is easily prepared, 
by passing sulphuretted hydrogen gas through water standing 
over powdered iodine, until the liquid, which, about the 
middle of the operation, becomes very brown on shaking, 
from solution of excess of iodine, is just decolorized again. 
Sulphur separates out abundantly, and can be removed either 
by filtering or decantation. The solution ought not to smell 
of sulphuretted hydrogen, or blacken paper dipped in acetate 
of lead or nitrate of silver, held over it. 

Hydriodic acid gas is very soluble in water, yielding a 
strongly acid liquid. The solution, colorless at first, soon 
becomes brown from liberation of free iodine. It may be 
restored to its original condition by adding solution of sul- 
phuretted hydrogen, and allowing sulphur to subside. 

HYDROCHLORIC ACID. 
Formula, HCl— 36.5. 

Hydrochloric acid is a volatile gas, which may be liberated 
from most of the salts termed chlorides by the action of sul- 
phuric acid. 

It is abundantly soluble in water, forming the liquid hydro- 
chloric or muriatic acid of commerce. The most concentrated 
solution of hydrochloric acid has a sp. gr. 1.2, and contains 
about 40 per cent, of gas ; that commonly sold is somewhat 
weaker, sp. gr. 1.14 to 1.16, containing about 28 per cent, real 
acid. 

Pure hydrochloric acid is colorless, and fumes in the air. 
The yellow color of the commercial acid depends upon the 
presence of traces of perchloride of iron, or of organic matter ; 
commercial muriatic acid also often contains free chlorine and 
sulphuric acid. 

HYDROGEN, PEROXIDE OF. 

Formula, H0=17. 

Tins curious compound, now more usually known under the 
name of hydroxyl, was discovered by Thenard in the year 
1818. It is a powerful oxidizing agent, and is also a reducing 
agent. In virtue of its former property, it has been recom- 



56 OUTLINES OF GENERAL CHEMISTRY. 

mended as a very convenient solution for converting the last 
traces of injurious liyposulphites in photographic prints into 
innocuous sulphates. This it does by oxidizing the hyposul- 
phites. But, unfortunately, it acts in two ways ; tirst it oxi- 
dizes, and then reduces them. 

HYDROSULPHUKIC ACID. 
Formula, HaS—Si. 

This gas, also known under the name of sulphuretted hydro- 
gen, is usually prepared by the action of dilute sulphuric acid 
on sulphide of iron. Cold water takes up about three times 
its volume of this gas, and the solution is slightly acid to 
litmus paper. In photography, it is chiefly used for reducing 
silver from the solutions of the salts of that metal. This it 
effects by throwing down sulphide of silver, which is again 
converted into the metal by fusion. 

It is most important that this substance should not be used 
in the dark room, as the fumes arising from it would destroy 
all chance of good photographic work with the salts of silver. 

IODINE. 
Symbol, 1=127. 

Iodine is chiefly j)repared at Glasgow, from kelp, which is 
the fused ash obtained on burning seaweeds. The waters of 
the ocean contain minute quantities of the iodides of sodium 
and magnesium, which are separated and stored up by the grow- 
ing tissues of the marine plant. 

In the preparation, the mother-liquid of kelp (which is the 
liquid that remains after most of the salts, which contain no 
iodine, have been separated by crystallization) is distilled at a 
gentle heat with a certain proportion of sulphuric acid and 
binoxide of manganese. 

The iodine sublimes in purple vapors, which condense to 
black crystals. 

Iodine has a bluish-black color and metallic lustre : it stains 
the skin yellow, and has a pungent smell, like diluted chlorine. 
It is extremely volatile when moist, boils at 347 deg. Fahr., 
and produces dense violet colored fumes, which condense in 
briliant plates. Specific gravity, 4.946. Iodine is very spar- 
ingly soluble in water, 1 part requiring 7000 parts for perfect 
solution -even this minute quantity, however, tinges the liquid 



IKON. 57 

of a brown color. Alcohol and ether dissolve it more abun- 
dantly, forming dark-brown solutions. Iodine also dissolves 
freely in solutions of the alkaline iodides, snch as the iodide 
of potassium, of sodium, and of ammonium. 

Iodine possesses the property of forming a compound of a 
deep blue color with starch. In using this as a test, it is nec- 
essary first to liberate the iodine (if in combination) by means 
of chlorine, avoiding an excess, or by means of nitric acid 
saturated with peroxide of nitrogen. The presence of alcohol 
or ether interferes to a certain extent with the result. 

mON, ACETATE OF, 
Formula, FeC2H302=115. 

There are two acetates of iron, a protacetate, which is 
nearly colorless, and a peracetate, which is red ; the former 
only is used in photography. 

A sohition of the protacetate containing a slight excess of 
the sulphate, but sufficiently pure for photographic purposes, 
may be made by dissolving 12 grains of protosulphate of iron 
and 12 grains of crystallized acetate of lead, each in half an 
ounce of water, mixing, and filtering from the white deposit, 
which is sulphate of lead. The solution is very unstable, and 
soon deposits a reddish subsalt, or if free acetic acid be pres- 
ent, it assumes a red color. 

A mixture of acetate of soda with protosulphate of iron, 
six grains of the former aud twelve grains of the latter to an 
ounce of water, acts in photography very much in the same 
manner as the pure solution of the acetate. 

mOISr, AMMONIO-CITRATE OF. 

This beautiful pharmaceutical preparation is a basic salt 
containing citric acid in union, both in ammonia and with per- 
oxide of iron. It is met with in the form of thin transparent- 
brown scales, produced by drying a syrupy solution on warm 
plates. 

The reaction of certain tests upon the ammonio-citrate, and 
also upon the ammonio-tartrate of iron, is peculiar. Free am- 
monia, which usually throws down a red sesquioxide from the 
persalts of iron, produces no deposit with these compounds; 
and ferrocyanide of potassium, which usually precipitates 
Prussian blue, simply produces a purple color; the mue pre- 



58 OUTLINES OF GENERAL CHEMISTKY. 

cipitate is, however, obtained on acidifying the liquid. The 
presence of the vegetable acid and the basic character due to 
ammonia are the causes of these anomalies, 

lEOlST, AMMONIO-SULPilATE OF. 

Formula, Fe(NH,)2(SOi)3 + 6 H20=392. 

This is a double salt of iron prepared by inixing equivalent 
proportions of protosulphate of iron and sulphate of ammonia. 
Take 139 parts of protosulphate of iron and 75 of sulphate of 
ammonia, dissolve them in a minimum of water, and set aside 
in an evaporating dish till the double salt crystallizes. It has 
been recommended to use this salt as a developer instead of 
the plain protosulphate, chiefly on account of its greater sta- 
bility. It possesses no other advantages. 

lEON, OXALATE OF (Fereous Oxalate). 

Formula, FeC204=144. 

This salt may be prepared by dissolving 2 ounces of ferrous 
sulphate in 30 ounces of water, and 396 grains of oxalic acid 
in 15 ounces of water, and mixing the two solutions. The 
ferrous oxalate is slowly precipitated. After which the clear 
liquid is decanted and the precipitate washed and dried. 

IKON, PROTOSULPHATE OF. ^ 

Formula, FeSO^ +7H20=2Y8. 

This salt, sometimes called copperas or green vitriol, is ob- 
tained by acting on iron wire or filings with dilute sulphuric 
acid, evaporating and crystallizing. When pure, the crystals 
are of a fine bluish -green color, free from red stains, and in 
the form of oblique rhombic j)risms. In dry air they efiioresce, 
but in moist air they become partially oxidized into a persalt 
and assume a red color. 

Protosulphate of iron forms double salts with the sulphates 
of ammonia and potash. Its aqueous solution absorbs binox- 
ide of nitrogen from the air and becomes of a deep brown 
color. As binoxide of nitrogen is itself a reducing agent, it 
has been erroneously supposed by some that this changed solu- 
tion is a more energetic developer than the plain pliotosul- 
phate. 



IRON. 59 

IRON, PERCHLORIDE OF. 

Formula, Fe2Cl6=325. 

There are two chlorides of iron, corresponding in composi- 
tion to the protoxide and the sesqnioxide, respectively. The 
protochloride is very soluble in water, forming a green solu- 
tion, which precipitates a dirty white protoxide on the addi- 
tion of an alkali. The perchloride, on the other hand, is 
dark brown, and gives with alkalies, a reddish-brown precipi- 
tate of peroxide. 

Perchloride of iron may be obtained in a state of purity by 
heating iron wire in excess of chlorine ; it condenses in the 
shape of brilliant and iridescent brown crystals, which are 
volatile, and dissolve in water, the solution being acid to test- 
paper ; a more easy mode of |)reparing the solution, however, 
is by digesting hydrochloric acid with excess of peroxide of 
iron. It is soluble in alcohol, forming the tinctura ferri ses- 
quichloridi of the pharmacopoeia. Commercial perchloride of 
iron ordinarily contains an excess of hydrochloric acid. 

IRON, IODIDE OF. 

Formula, Fel2==310. 

Iodide of iron is prepared by digesting an excess of iron fil- 
iiigs with pulverized iodine and water. It is very soluble in 
water and in alcohol, but the solution rapidly absorbs oxygen, 
and deposits peroxide of iron ; hence the importanoe of pre- 
serving it in contact with metallic iron, with which the 
separated iodine may recombine. By very careful evaporation, 
hydrated crystals of protoiodide may be obtained, but the com- 
position of the solid salt usually sold under that name cannot 
be depended on. 

IRON, SESQUIOXALATE OF (Ferric Oxalate). 

Formula, ¥e,{C20,)s=S76. 

This salt is prepared by dissolving the hydrated peroxide in 
a solution of oxalic acid. It is very soluble in water, and has 
several applications in photography, based upon its sensitive- 
ness to light, by which it is reduced to the ferrous oxalate. 
Its chief use is in the platinotype printing process, in which 
paper, prepared with a mixture of ferric, oxalic and potassic 



60 OUTLINES OF GENERAL CHEMISTRY. 

cbloroplatinite is exposed to light under a negative, when an 
image is formed in ferrous oxalate. The picture is floated on 
a hot solution, consisting chiefly of neutral potassic oxalate, by 
which the ferrous oxalate is dissolved, and acting instantly 
upon the platinic salt with which it is in contact, the latter is 
reduced, and an image formed in metallic platinum. 

KAOLIN, OR CHINA CLAY. 

This is employed by photographers to decolorize solutions 
of nitrate of silver which have become brown from the action 
of albumen or other organic matters. 

LEAD, ACETATE OF. 
Pb(C,H302),+H30=343. 

The neutral acetate of lead is a very abundant substance in 
commerce, and is known as sugar of lead. It is prepared by 
digesting oxide of lead in pyroligneous or acetic acid, and 
crystallizes in acicular masses. 

Acetate of lead is easily soluble in cold water, but the solu- 
tion is usually milky, either from the presence of a little car- 
bonate mixed with the acetate, or from carbonic acid or car- 
bonate of lime in the water used. 

When a little of the salt is added to the gallic acid developer 
for paper negatives or positives, it exercises a marked effect in 
forwarding the development. The rationale is not known,, 
but the fact remains. 

LIME, CHLORIDE OF. 

The exact chemical composition of this substance is a subject 
of dispute among chemists. It emits the peculiar odor of 
hypochlorous acid when exposed to the air, and at the same 
time absorbs carbonic acid. 

In photography it is sometimes used in the gold toning bath 
to neutralize the acidity of the solution; but it must be.sO' 
used with great discretion, otherwise the chlorine which 
escapes attacks the silver image and greatly weakens it. 

On account of its pow^erful oxidizing properties, a solution 
of this salt has been recommended for eliminating the last traces 
of soluble hyposulphite from washed photographic prints. 
This it effects in the same way as peroxide of hydrogen, by 



LITMUS. 61 

oxidizing the hyposulphite into an innocuous sulphate. It is 
doubtful, however, whether the advantage gained will counter- 
balance the disadvantage of an enfeebled print. 

The so-called chloride of lime is an excellent substance for 
removing silver stains from the hands, linen, etc. Make up a 
little dry chloride into a paste, with water acidulated with any 
acid, and apply to the stains by hard rubbing. They will 
quickly disappear. Afterwards wash with hyposulphate of 
soda solution, which absorbs the disagreeable chlorine fumes. 

LITHIUM, IODIDE OF. 

Formula, Li + 6 Aq=2 i2. 

Lithium is one of the rarer elements, found in the mineral 
kingdom, and the basic reactions of its oxide are so strong 
that it takes its place with the alkalies potash and soda. 
Metallic lithium, though a metal, is the lightest solid-body 
known, its density being little more than one-half of that of 
water. 

Iodide of lithium has been proposed for photographic use, 
being more easily soluble in alcohol than iodide of potassium. 
Its deliquescent nature, however, is an objection, as also is the 
difficulty and expense of obtaining it commercially in a pure 
state. A solution of the iodide may be prepared by mixing 
equivalent proportions of crystallized sulphate of lithia and 
iodide of calcium in concentrated aqueous solution evaporating 
to dryness over sulphuric acid m vacuo, and exhausting the 
dry residue with alcohol of .805. Iodide of calcium is j)re- 
f erable to iodide of barium, in consequence of the latter being 
more frequently contaminated with an excess of base. 

LITMUS. 

Litmus is a vegetable substance prepared from various 
lichens, which are principally collected on rocks adjoining the 
sea. The bine coloring-matter is extracted by a peculiar pro- 
cess, and is afterwards made up into a paste with chalk, plaster 
ot^Paris, etc. 

Litmus occurs in commerce in the form of small cubes of a 
fine violet color. In using it for the preparation of test-papers, 
it is digested in hot water, the solution concentrated at a gentle 
heat, and sheets of porous paper soaked in the blue liquid so 
formed. The red papers are prepared at first in the same man- 



62 OUTLINES OF GENERAL CHEMISTRY. 

ner, but are afterwards placed in water which has been 
rendered faintly acid with sulphuric or hydrochloric acid. 
Papers are prepared also of a purplish tint, which becomes full 
blue with alkalies, and bright red with acids. 

MAGNESIUM. 

Symbol, Mg=24. 

This is a silver white metal of crystalline structure, and some- 
what brittle. It is obtained from the chloride by several 
methods, which need not be described here. 

If kept in dry air this metal is not altered, but, in damp 
air, it soon becomes covered with a film of hydrate of mag- 
nesiuna. 

Its only use in photography is for the purpose of illumina- 
tion, as it emits a most dazzling light when burnt in air or 
oxygen. For this purpose it is sold in the form of wire or 
ribbon. The light emitted is supposed to be more actinic than 
any other form of artificial illumination. 

An ingenious lamp with a reflector has been devised, which 
facilitates the production of a regular and constant flame 
from the burning metal. The product of the combustion is 
magnesia. 

The bromide and iodide of magnesium are objectionable in 
photography by reason of their great affinity for water, and 
their liability to decomposition. 

MERCURY, CHLORIDE OF. 

(Formerly BiCHLOEroE). 

Formula, IIgCl2=271. 

This salt, also called corrosive sublimate, and often bichloride 
of mercury (the atomic weight of mercury being doubled), 
maybe formed by heating mercury in excess of chlorine, or, 
more economically, by subliming a mixture of sulphate '■ of 
mercury and chloride of sodium. 

Projperties. — A very corrosive and poisonous salt, usually 
sold in semi-transparent, crystalline masses, or in the state of 
powder. Soluble in 16 parts of cold, and in 3 of hot water ; 
more abundantly so in alcohol, and also in ether. The solu- 



NITRIC ACID. 63 

bility in water may be increased by the addition of free hydro- 
chloric acid, or of chloride of ammonium. 

This salt is sometimes used as an intensilier for negatives. 
A saturated aqueous solution is poured over the image until 
the latter becomes of a whitish gray color. The film is then 
washed, and treated with a one-grain sohition of iodide of 
potassium until tlie image assumes a green tone, which is very 
non-actinic. Negatives so treated, after repeated exposures to 
light, become too dense. 

NAPHTHA. 

This name is applied to several liquids sold in commerce* 
First, wood naphtha, often used for burning in lamps, which 
is the same as pyroxylic spirit or methylic alcoliol, q. v. / 
second, coal naphtha, a volatile liquid wliich distills over in 
the process of manufacturing gas ; and, thirdly, mineral naph- 
tha, found in the soil of certain^laces in Europe and Asia, 
and very recently abundantly in Canada and Pennsylvania. 

The two latter substances, to which the name of naphtha 
ought to be restricted, possess the same leading characteristic, 
in that they are hydrocarbons, burning with a bright, smoky 
flame ; they do not mix with water, and they dissolve caout- 
chouc, in all which respects they differ from " wood naphtha." 

NITRIC ACID. 
Formula, HN03=63. 

Nitric acid, or aqua-fortis^ is prepared by adding sulphuric 
acid to nitrate of potash, and distilling the mixture in a retort. 
Sulphate of potash and free nitric acid are formed, the latter 
of which, being volatile, distils over in combination with one 
atom of water previously united with the sulphuric acid. 

Anhydrous nitric acid is a solid substance, white and crys- 
talline, but it cannot be prepared except by an expensive and 
complicated process. 

The strongest liquid nitric acid contains one atom of water, 
and has a specific gravity of about 1.5 ; if perfectly pure, it is 
colorless; but usually it has a slight yellow tint, from partial 
decomposition into peroxide of nitrogen ; it fumes strongly in 
the air. 

The strength of commercial nitric acid is subject to much 
variation. An acid of sp. gr. 1.42, containing about four 



64 OUTLINES OF GENERAL CHEMISTRY. 

atoms of water, is often met with. If the specific gravity is 
much lower than this (less than 1.36), it will scarcel)'^ be 
adaj3ted for the preparation of pyroxyline. The yellow ni- 
trous acid, so called, is a strong; nitric acid, partially saturated 
with the brown vapors of peroxide of nitrogen ; it has a high 
specific gravity — usually about 1.4.5 ; but this is deceptive, 
being caused in part by the presence of the peroxide. On 
mixing with sul])hnric acid, tlie color disappears, a compound 
being formed which has been termed a sulphate of nitrous 
acid. 

Kitric acid is a powerful oxidizing agent ; it attacks all the 
common metals, with the exception of gold and platinum. 
Metals dissolving in acid usually derive the necessary amount 
of oxygen from the water, hydrogen being given off ; but 
when nitric acid is the solvent, oxygen is derived from it, and 
N2O2 or N2O3 or ISTgOi escapes. Animal substances, such as 
the cuticle, nails, etc., are tinged of a permanent yellow color, 
and are deeply corroded by a prolonged application. Nitric 
acid forms a numerous class of salts, all of which are soluble 
in water ; hence its presence cannot be determined by any 
precipitating reagent, in the same manner as that of hydro- 
chloric and sulphuric acid. 

The principal impurities in commercial nitric acid are 
chlorine and sulphuric acid ; also peroxide of nitrogen, which 
tinges the acid yellow, as already described. Chlorine is de- 
tected by diluting the acid with an equal bulk of distilled 
water, and adding a few drops of nitrate of silver — a milki- 
ness, which is chloride of silver in suspension, indicates the 
presence of chlorine. In testing for sulphuric acid, dilute the 
nitric acid as before, and drop in a single dro23 of solution of 
chloride of barium ; if sulphuric acid be present, an insolu- 
ble precipitate of sulphate of baryta will be formed. 

A convenient form of acid for preparing pyroxyline in 
the nitric acid of 1.45. The question has been asked, why so 
concentrated an acid is recommended, seeing that it is after- 
wards to be diluted with water ? There are two reasons ; first, 
because this acid is cheaper in the end, and perhaps more uni- 
form than a weaker acid ; and secondly, it is important that 
both the sulphuric and nitric acid should be as strong as pos- 
sible, in order to allow of the use of suflicient water to raise 
the temperature of the resulting nitro-sulphuric acid at once 
to the proper point, and so to ol)viate the necessity of em- 
ploying artificial heat. 



NITEO-HYDROCHLORIC ACID. 65 

NITKOGEN. 

Symbol, N=14. 

Nitrogen is an element, existing as a gas, in the free state. 
Our atmosphere consists of a mixture of the two elements 
nitrogen and oxygen, in the proportion of four volumes of 
the first with one volume of tlie second. 

Nitrogen, when free, is remarkably devoid of sensible prop- 
erties, and can scarcely be made directly to unite with any 
element. Its compounds are, however, numerous and import- 
ant, nitric acid, HNO3, being one of its oxides; ammonia, 
NH3, is its compound with hydrogen, and cyanogen, CN, its 
compound with carbon. 

NITRO-GLUCOSE. 

When 3 fluid ounces of cold nitro-sulphuric acid, consist- 
ing of 2 ounces of oil of vitriol and 1 ounce of highly con- 
centrated nitric acid, are mixed witli 1 ounce of finely pow- 
dered cane sugar, there is formed at first a thin, transparent, 
pasty mass. If it is stirred with a glass rod for a few minutes 
without interruption, the paste coagulates as it were, and sep- 
arates from the liquid as a thick tenacious mass, aggregating 
into lumps, which can easily be removed from the acid mix- 
ture. 

This substance is a substitution compound, and is derived 
from sugar as gun-cotton is from cotton, by the displacement 
of a part of the H in sugar by NO2. It has a very acid and 
intensely bitter taste. Kneaded in warm water until the lat- 
ter no longer reddens litmus paper, it acquires a silvery color 
and a beautiful silky lustre. When dissolved in collodion, it 
ozonizes the ether on keeping for some months, and hence 
theje is a rapid liberation of iodine when the iodizer is added. 
Alkalies decompose nitro-glucose, evolving a smell of burnt 
sugar. The product lessens the sensitiveness of collodion, but 
increases the intensity of the image. 

NITRO-HYDROCHLORIC ACID. 

This liquid is the aqua regia of the old alchemists. It is 
produced by mixing nitric and hydrochloric acids ; the oxy- 
gen of the former combines with the hydrogen of the latter, 
forming water and liberatino; chlorine. 



bb OUTLINES OF GENERAL CHEMISTRY. 

The presence of free cliloriiie confers on tlie mixture the 
power of dissolving gold and platinum, which neither of the 
two acids possesses separately. In preparing aqua regia it is 
usual to mix one part, by measure, of nitric acid with four of 
hydrochloric acid, and to dilute with an equal bulk of water. 
The application of a gentle heat assists the solution of the 
metal ; but if the temperature rises to the boiling point, effer- 
vescence and loss of chlorine takes place. 

ORGANIC MATTER. 

This term is often employed to designate varieties of non- 
volatile vegetable and animal substances of unknown composi- 
tion, which are prone to change by absorption of oxygen. 
Bodies which have a definite formula, however, are often 
included in the class of " organic matters " if they easily pass 
by oxidation into ill-defined products. Thus the sugar of 
liquorice, which soon decomposes and becomes brown, may be 
referred to under that head ; but a stable vegetable acid, like 
acetic acid, or a neutral substance not readily oxidizable, such 
as glycerine, although really organic in composition, would 
not usually be referred to by this term. 

Organic matter of the kind above alluded to reacts upon 
nitrate of silver in presence of light, reducing it more or less 
perfectly to the metallic state, and becoming itself oxidized. 

Carbon is essentially the organic element, as every organic 
body is a compound of carbon. 

OXYGEN. 

Symbol, 0=16. 

Oxygen gas is one of the elements ; it is very abundantly 
distributed, both in the free state and in combination. In the 
former state, it exists in our atmosphere, but mixed with four 
times its bulk of the elementary gas iiitrogen. Its compounds, 
as stated in Chapter I., are either acids, bases, or neutral 
bodies. It can be obtained in a state of almost perfect purity 
and in great abundance by heating the salt chlorate of potash 
to gentle redness in a flask or retort ; chloride of potassium 
remains. This decomposition is rendered more easy by first 
mixing the powdered chlorate with one-fourth of well-dried, 
and powdered, black oxide of manganese. 



PHOSPHORIC ACID. 67 

OZONE. 

Oxygen, in tlie state in which it usually exists in the at- 
mosphere, exhibits no very powerful chemical properties at 
ordinary temperatures ; thus, it has no action on iodide of 
potassium — it cannot displace the iodine to combine with the 
potassium. It appears, however, that oxygen, though an ele- 
ment, is capable of taking on a more active form, in which 
condition it almost resembles chlorine in its tendency to com- 
bine with bodies. In this state, it instantly attacks iodides of 
potassium, sodium, etc., forming potash, soda, etc., and setting 
iodine free. To this modification of oxygen the term ozone 
has been applied, a name derived from oC&? {ozo, I smell), on 
account of its having a remarkable odor. 

Some organic bodies, as ether and spirits of turpentine, have 
the power of converting a portion of the oxygen of the air 
into ozone, which they loosely retain. This condition of ether 
can easily be produced by thrusting a red-hot wire* into the 
vapor in a bottle containing a little ether. On shaking the 
bottle afterwards, and testing with a solution of iodide of 
potassium, ozone will be indicated by the liberation of iodine. 
This condition of ether occurs spontaneously after a time ; it 
may be got rid of by distillation from solid potash. 



PHOSPHORIC ACID. 

Formula, HPO3=80. 

This acid, in a diluted form, is used in Willis's aniline pro- 
cess. It may be prepared by pouring 4 fluid ounces of 
nitric acid and 8 ounces of distilled water on 6 drams of 
phosphorous in a retort, and applying the heat of a sandbath. 
The distillation should proceed till the residue in the resort is 
of a syrupy consistence. The syrup is then poured into a 
platinum vessel, and heated to a dull red. On cooling, it con- 
cretes into a transparent mass, often assuming the crystalline 
form. This constitutes what is called glacial phosphoric acid, 
which may be diluted to au}^ extent. 

* No flajiie should ever be brought near an ether bottle, for fear of an 
explosion. 



68 OUTLINES OF GENERAL CHEMISTRY. 

POTASH. 
Formula, KHO=56.1. 

Potash, the oxide of potassium, is obtained from carbonate 
of potash by separating the carbonic acid by means of caustic 
lime. Lime is a more feeble base than potash, but the carbon- 
ate of lime, being insoluble in water, is at once formed on ad- 
ding milk of lime to a solution of carbonate of potash in not 
less than twelve parts of water. 

Properties. — Usually met with in the form of solid lumps, 
or in cylindrical sticks, which are formed by melting the 
potash and running it into a mould. It always contains one 
atom of water, which cannot be driven off by the application of 
heat. 

Potash is soluble almost to any extent in water, much heat 
being evolved. The solution is powerfully alkaline, and acts 
rapidly upon the skin ; it dissolves fatty and resinous bodies, 
converting them into soaps. Solution of potash absorbs car- 
bonic acid quickly from the air, and should therefore be pre- 
served in stoppered bottles ; the glass stoppers must be wiped 
occasionally, in order to prevent them from becoming im- 
movably fixed by the solvent action of the potash upon the 
silica of the glass. 

The liquor potassse of the London Pharmacopoeia has a sp. 
gr. of 1.063, and contains about five percent, of real potash. 
It is usually contaminated with carbonate of potash, which 
causes it to effervesce on the addition of acids ; also, to a 
less extent, with sulphate of potash, chloride of potassium, 
silica, etc. 

POTASH, BICHROMATE OF. 
Formula, \^^Q\\0,='>.^^.<6. 

This salt is largely manufactured for the use of calico print- 
ers, from a native compound of the oxides of chronium and 
iron. It occurs in fine orange-colored crystals, which are 
soluble in about ten parts of water at 60 degs. Fahr. 

There are two chromates of potash — a neutral chromate 
which is yellow and contains an atom of each constituent ; and 
a bichromate, orange-red, as before mentioned, and having two 
atoms of acid to one of base. The chromic acid of bichromate 
of potash, in contact with organic bodies, such as gelatine, is de- 



POTASH. 69 

composed by liglit, yielding up half its oxygen to the organic 
body, and being itself reduced to a lower oxide of chronium. 
The organic body thus oxidized, if previously soluble in 
water, is often, as in the case of gelatine, rendered insoluble 
in water, and upon this fact are founded numerous photographic 
processes. 

POTASH, CARBONATE OF. 
Formula, K2,C03=13&.2. 

The impure carbonate of potash, termed pearlash, is ob- 
tained from the ashes of wood and vegetable matter, in the 
same manner as carbonate of soda used to be prepared from 
the ashes of seaweeds. Salts of potash and of soda appear 
essential to vegetation — the former to land and the latter to 
sea plants — and are absorbed and appropriated by the living 
tissues of the plants. They exist in the vegetable structure 
combined with organic acids in the form of salts, oxalate, tar- 
trate, etc., which, when burned, are converted into carbonate. 

Properties. — The pearlash of commerce contains large and 
variable quantities of chloride of potassium, sulphate of potash, 
etc, A purer carbonate is sold, which is free from sulphates, 
and with only a trace of chlorides. Carbonate of potash is a 
strongly alkaline salt, deliquescent, and soluble in twice its 
weight of cold water; insoluble in alcohol, and employed to 
deprive it of water. 

POTASH, NITATE OF. 
Fomula, KNO3=101.1. 

This salt, also termed nitre, or saltpetre, is an abundant nat- 
ural product, found efflorescent upon the soil in certain parts 
of the East Indies. It is also produced artificially in what are 
called nitre-beds. 

There are different qualities of nitre sold in commerce, 
some of which contain much chloride of potassium, detected 
on dissolving the nitre in distilled water, and adding a drop 
or two of solution of nitrate of silver. This impurity is in- 
jurious when the nitre is employed for photographic use : in 
the manufacture of pyroxyline it decomposes the nitric acid ; 
and in the case of positive developing solutions, the presence 
of chloride in the nitre seems to produce a white cloudiness, 
on the film. 



70 OUTLINES OF GENERAL CHEMI8TKY. 

A quality of nitre which answers very well for making py- 
roxyline can be obtained at the operative chemists' at 35 
cents per pound ; it is often sold as pure nitre, but usually 
contains sufficient chloride to produce an opalescence with 
nitrate of silver ; if the impurity is in larger quantity, and 
produces a decided precipitate with nitrate of silver, the sam- 
ple must be rejected. 

POTASSIUM, BEOMIDE OF. 
Formula, KBr=ll9.1. 

Bromide of potassium is prepared by adding bromine to 
caustic potash, and heating the product, which is a mixture of 
bromide of potassium and bromate of potash, to redness, in 
order to drive off the oxygen from the latter salt. It crystal 
lizes in anhydrous cubes, the chloride and iodide of potassium ; 
it is easy soluble in w«.ter, but very sparingly so in alcohol ; it 
yields red fumes of bromine when acted upon by hot and 
strong sulphuric acid. 

This salt is useful in photographic processes on paper, but 
in collodion it is liable to cause turbidity and spots. 

POTASSIUM, CYANIDE OF. 

Formula, KCN— 55.1. 

Cyanide of potassium may be regarded either as a com- 
pound of cyanogen and potassium (see cyanogen), or as a 
salt, derived from hydrocyanic, or " prussic " acid. 

It is obtained from ferrocyanide of potassium by mixing 
eight parts of this salt, thoroughly dried, with three of dried 
carbonate of potash, and fusing in a covered earthen crucible. 
When effervescence has ceased, and iron, which separates, has 
settled down, the clear liquid is poured off, and on cooling 
solidifies into the white mass known as cyanide of potassium. 
Thus obtained it is not quite pure, and, as sold in commerce, 
it often only contains about half its weight of true cyanide ; 
but the impurities present, of which carbonate of potash is 
the principal, do not produce any injurious effect beyond less- 
ening the strength of the salt. It is highly poisonous, and, if 
treated with an acid, it gives off the very volatile and poison- 
ous hydrocyanic acid, the vapor of which, incautiously in- 
haled, might cause fainting, or even death. 



POTASSIUM. 71 

POTASSIUM, FLUORIDE OF. 

Formula, KF=58.1. 

Preparation. — Fluoride of potassium is formed by saturat- 
ing hydrofluoric acid with potash, and evaj)orating to dryness 
in a platinum vessel. Hydrofluoric acid contains the element 
fluorine combined with hydrogen ; it is a powerfully acid and 
corrosive liquid, formed by decomposing fluor spar, which is a 
fluoride of calcium, with strong sulphuric acid ; the action 
which takes place being precisely analagous to that involved 
in the preparation of hydrochloric acid. 

CaF.+H2SO,=CaSO,+2HF. 

Properties. — A deliquescent salt, occurring in small and im- 
perfect crystals. Yery soluble in water ; the solution acting 
upon glass in the same manner as hydrofluoric acid. 

POTASH, OXALATE OF. 

Formula, KgCjO^. 

Potassium oxalate, or the neutral oxalate of potash, which is 
now much employed in photography, as a solvent of ferrous 
oxalate,* for use as a developer, is prepared by neutralizing a 
solution of oxallic acid with potassium carbonate, and then 
evaporating and crystallizing. It is soluble in three parts of 
water. Those photographers who prepare it for their own 
use rarely trouble themselves with crystallizing it, but allow it 
to remain in the liquid form. 

POTASH, PERMANGANATE OF. 

Formula, K3Mn208=316.2. 

Is a salt which has been recommended for destroying or- 
ganic matter in the nitrate of silver bath. It acts as an 
oxidizing agent. 

POTASSIUM, IODIDE OF. 

Formula, KI=166.1. 

This salt may be prepared by dissolving iodine in a solution 
of potash until it assumes a brown color. Iodide of potassium 



t2 OUTLINES OF GENERAL CHEMISTRY. 

and iodate of potash are thus formed, bnt by evaporation and 
heating to redness the latter salt parts with its oxygen, and is 
converted into iodide of potassium. The salt is then redis- 
solved, filtered, and allowed to crystallize. 

Iodide of potassium may contain several incidental impuri- 
ties from carelessness in the manufacture. If carbonate, 
iodate, or sulphate be present, solution of chloride of barium 
will detect them. In the two former instances, the precipitate 
dissolves on the addition of a single drop of pure dilute nitric 
acid, but, in the latter case, it is insoluble. 

A simple test for carbonate of potash, which is the most 
objectionable impurity, is to dissolve the iodide in about three 
times its weight of lime water; a turbidity will indicate the 
presence of carbonate. 

The mere fact of reddened litmus paper becoming blue in 
solution of iodide of potassium is no proof of impurity, since 
the finest crystals which can be obtained have an alkaline re- 
action. But if an alcoholic solution of the iodide remains 
quite colorless when exposed for several days to a strong light, 
it is almost certain that an excess of alkali is present ; the 
chemically pure iodide of potassium is generally decomposed 
by light, and assumes a faint straw-yellow tint, returning, 
however, to its colorless condition on putting the bottle again 
in a dark place. 

Iodide of potassium may, when required, be purified by re- 
crystallizing it from spirit, or by dissolving it in alcohol of 
.805, in which carbonate, sulphate, and iodate are insoluble. 

POTASSIUM, SULPHIDE OF. 

There are many sulphides of potassium, but the one com- 
monly employed by photographers, and sold in commerce as 
" liver of sulphur," is an impure tersulphide. It is prepared 
by heating sulphur with carbonate of potash, the result of 
which is that a portion of the sulphur is oxidized into sul- 
phuric acid, and combines with potash, forming sulphate of 
potash, whilst another portion enters into combination with 
potassium, producing a sulphide of potassium containing three 
atoms of sulphur to one of potassium. 

Sulphide of potassium is used for reducing the silver from 
old hyposulphite of soda fixing solutions. 

POTASSIUM, SULPHOCYANIDE OF. 

{See SULPHOCYANIDES.) 



SILVER. 73 

PYROGALLIC ACID. 
Formula, H3C6ll303=126. 

This substance is obtained by the action of heat of 420 deg. 
Fahr., on gallic acid. The acid sublimes and is collected in 
the form of white shiny scales. 

Pyrogallic acid (sometimes called pyrogallol) can hardly be 
called a real acid, as it does not redden blue litmns ]3aper. It 
is very soluble in water, alcohol, and ether. Its aqueous solu- 
tion soon decomposes by oxidation ; but an alcoholic or ether- 
eal solution will remain unchanged for a very long time. 

It is a powerful deoxidizer, and reduces the oxides of all 
the noble metals, hence its value as a developer in photog- 
raphy. 

PYHOXYLIC SPIRIT. {See Alcohol, Methylic.) 
PYEOXYLINE. 

The name pyroxyline is applied to a series of compounds, 
obtained by the action of mixed nitric and suljDhuric acids on 
vegetable fibres ; they are all more or less ex]3losive, the 
strongest being gun cotton ; the soluble varieties are termed 
sometimes collodion wool. They are all substitution com- 
pounds, being derived from the vegetable fibre by the substi- 
tution of atoms of NOg for an equal number of atoms of H 
in the fibre. 

Photographic pyroxyline is prepared with hot acids, heat 
being found remarkably to modify the products, which, thus 
prepared, have not yet been thoroughly analyzed. A mix- 
ture of acids which, if cold, would give an insoluble cotton, 
will, when hot, often produce a perfectly soluble product; 
and another acid mixture which would yield a pyroxyline giv- 
ing a very viscid collodion with four grains to the ounce, will, 
when used hot, give a product soluble to the extent of seven 
or eight grains to the ounce, and yet the solution will be quite 
fluid. 

SEL D'OR. {See Gold, Hyposulphite of.) 

SILVEK. 

Symbol, Ag=108. 

This metal, the Luna or Diana of the alchemists, is found na- 
tive in Peru and Mexico ; but its principal ore is the sulphide. 



Y4 OUTLINES OF GENERAL CHEMISTKY. 

When pui-e it has a sp. gr. of 10.5, and is very malleable 
and ductile ; melts at a bright red heat. Silver does not oxi- 
dize in the air, but when exposed to an impure atmosphere, 
containing traces of sulphuretted hydrogen, it is slowly tar- 
nished, from formation of sulphide of silver. It dissolves in 
strong boiling sulphuric acid, but the best solvent for it is 
nitric acid. 

The standard coin of the realm is an alloy of silver and 
copper, containing 92.5 per cent, of silver. 

To prepare pure nitrate of silver from it, dissolve in nitric 
acid by aid of heat, and evaporate until crystals are obtained. 
Then wash the crystals with a little dilute nitric acid, redis- 
solve them in water, and crystallize by evaporation a second 
time. 

The process is also occasionally conducted by boiling down 
the impure acid solution of the silver to dryness, without any 
crystallization, and fusing the product pretty strongly, until a 
portion taken out, dissolved in water, and filtered from oxide 
of copper, ceases to give a blue color with ammonia, showing 
that tiie nitrate of copper is quite decomposed ; afterwards, 
recrystallize as before. Perhaps the easiest method is to dis- 
solve the alloy in nitric acid, immerse a piece of metallic cop- 
per until the silver is wholly precipitated, remove the copper, 
wash the silver well with water, tlien with a little nitrate of 
silver to remove any adhering copper, and lastly dissolve in ni- 
tric acid and crystallize ; or else take the silver as precipitated by 
copper, wash it and dissolve in nitric acid, avoiding all excess 
(or if excess has been added, evaporate carefully to dryness to 
expel it) ; and to neutralize and remove traces of copper, add 
oxide of silver to the boiling solution until on filtering and 
testing a portion with ammonia, no blue color is perceived, 
then filter the whole. This solution, if of the right strength 
(see appendix), might be used at once for photographic pur- 
poses. 

SILVER, ACETATE OF. 

Formula, AgCHgO^^lGY. 

This is a difficultly soluble salt, deposited in white lamellar 
crystals when an acetate is added to a strong solution of ni- 
trate of silver. 

It is sometimes used as an addition to the negative nitrate 
bath, but as the commercial acetate of silver is often impure 



SILVER. T5 

and contaminated with carbonate of silver — which would make 
the bath alkaline — it is better to use the acetate of soda, ac- 
cording to the directions to be given afterwards. 



SILYER, ALBUMINATE OF. 

This name has been given to the insoluble white substance 
precipitated on adding nitrate of silver to a solution of albu- 
men, and which analysis shows to contain oxide of silver com- 
bined with the animal matter. It is reducible to a red sub- 
compound, both by white light and by hydrogen gas. 



SILVER, AMMONIO-NITRATE OF. 

Crystallized nitrate of silver absorbs an moniacal gas rapidly, 
with production of heat sufficient to f'u^Q the resulting com- 
pound, which is white and contains 22 .3 pjr cent, of ammonia 
with 77.5 per cent, of nitrate of silvjr TJi ' solution how- 
ever which photographers employ, is pre^a eJ by adding to 
the solution of nitrate of silver, quite ii ; itral, a pure solution 
of ammonia until the precipitate which first forms is nearly 
redissolved, and then filtering. 

Ammonio-nitrate of silver is often used for sensitizing plain- 
salted paper for positive printing; but it is not suited to albu- 
minized paper, as the ammonia dissolves off the albumen. 



SILYER, BROMIDE OF. 

AgBr=188. 

This salt is now much used in photography, especially in the 
■dry process. 

It may be produced, either by direct union of its elements, 
as, in the Daguerreotype process, or by double decomposition 
between nitrate of silver and a soluble bromide, as in the wet 
and emulsion processes of photography. 

Bromide of silver is insoluble in water : soluble in alkaline 
hyposulphites, cyanides, sulphocyanides, and ammonia. By 
exposure to light it darkens to a tawny red color ; yet, like the 
corresponding iodide, it is capable of yielding, by development, 
& visible from an invisible imasre in the camera. 



^6 OUTLINES OF GENERAL CHEMISTET. 

SILVER, CARBONATE OF. 

Formula, Ag2,C03=27G. 

This is a white or yellowish powder deposited on adding 
any soluble carbonate to solution of nitrate of silver. It 
is only slightly soluble in water, or in solution of nitrate of 
silver, but yet sufficiently so to produce an alkaline reaction 
to litmus. More soluble in water containing nitrate of ammo- 
nia, and freely so in ammonia itself. Also dissolved by dilute 
nitric or acetic acid, forming a nitrate or acetate. 

SILYER, CHLORIDE OF. 
Formula, AgCl=- 143.5. 

Chloride of silver may be obtained by the direct union of 
silver and chlorine, or, as in the preparation of papers for 
positive printing, by double decomposition of nitrate of silver 
and an alkaline chloride. 

On mixing the solutions, the chloride of silver falls as a 
curdy white precipitate, insoluble in water and nitric acid, very 
soluble in alkaline hyposulphites, cyanides, and sulphocyanides^ 
and in ammonia even when dilute. On exposure to light 
it goes through various shades of violet until it becomes finally 
black. 

SILVER, CITRATE OF. 

' Formula, Ag3C6H50,=-513. 

A white salt precipitated on adding a soluble citrate to so- 
lution of nitrate of silver. Its property of being reduced to 
a colored sub-salt by the action of light, or by a deoxidizing 
agent, renders it useful in photography. 

Citric acid added to solution of nitrate of silver produces, 
no precipitation. This acid is what is termed tribasic, *'. 6. 
an acid, one atom of which combines with three atoms of 
base to form a neutral salt. 

SILVER, FLUORIDE OF. 

Formula, AgF^127. 

This compound differs essentially from the other silver hal- 
oids in being soluble in water. The dry salt fuses on being 



SILVER. 77 

heated, and is reduced by a higher temperature, or by expos- 
ure to light. 

SILVER, HYPOSULPHITE OF. 

Formula, Ag2S203=328. 

This salt maybe obtained by adding a dilute solution of ni- 
trate of silver to an excess of concentrated hyposulphite of 
soda; it is thrown down, mixed with a little sulphide of silver, 
from which it is separated after washing by ammonia, which 
dissolves the hyposulphite of silver, and from which it is pre- 
cipitated by exact neutralization with nitric acid. 

White, nearly insoluble, and tastes sweet ; it is very unstable, 
easily decomposing into sulphide of silver and sulphuric acid. 

It forms two double salts with hyposulphite of soda : one is 
very soluble in water; the other salt is sparingly soluble. 
These salts when in solution, are precipitated by iodide of po- 
tassium, but not by chloride of sodium. They are far more 
stable than the simple hyposulphite. Taste, exceedingly sweet. 

SILVER, IODIDE OF. 
Formula, Agl=235. 

Iodide of sillver in the Daguerreotype process is produced 
by the direct union of its element, but in the wet processes of 
photography by double decomposition of nitrate of silver and 
a soluble iodide. 

Obtained by mixing solutions of nitrate of silver and an 
alkaline iodide, it forms a yellow precipitate. 

This compound differs from the chloride and bromide of 
silver in being insoluble in ammonia, but it resembles them in 
being soluble in alkaline hyposulphites, cyanides, and sulpho- 
<jyanides, and to some extent in concentrated solutions of the 
alkaline chlorides, bromides and iodides. Heat changes it 
temporarily to deep yellow. Light turns it brown. 

SILVER, NITRATE OF. 

Formula, Ag,N03= 170. 

Prejparation and Properties. — The preparation of nitrate 
of silver has been sufficiently described under the head of 



78 OUTLINES OF GENERAL CHEMISTRY. 

Silver, to which the reader is referred. There are, however, 
some points of practical importance yet to be consid.ered.^%^ 

Pure nitrate of silver may be made from alloys of silver 
and copper, as described (art. Silver) ; but since the heat 
which is necessary to decompose the nitrate of copper often 
produces nitrite of silver, it is advised that silver nearly free 
from copper should be chosen in preference. The consump- 
tion of nitrate of silver in photography has now become 
very large, and it fortunately happens that the crystallized salt 
can be obtained in almost any quantity at a moderate price, 
being a bye-product in the operations of parting gold and sil- 
ver, which are carried on in the refineries. The assay processes 
also yield a portion of the nitrate sold in commerce, but not 
by any means the greater part. 

This facility of obtaining commercial crystallized nitrate of 
silver at a price which is very little above that of the metal it 
contains, has, however, acted injuriously as regards the purity 
of the article, since it necessarily leaves its manufacture in 
the hands of a few individuals, who are not able to pay that 
attention to it which is needed. The crystals are usually sent 
out simply dried off from the nitric acid, as a refuse product 
on which no profit can be expected. Intentional adulteration, 
however, is not practiced by the lai-ge producers of nitrate of 
silver, as far as the author is aware. 

The purity of nitrate of silver may easily be ascertained by 
dissolving a portion in distilled water, and precipitating the 
solution entirely with pure hydrochloric acid ; the liquid 
filtered from the precipitate should leave no residue on evapor- 
ation to dryness. 

Nitrate of potash and nitrate of copper have been spoken 
of as occasional impurities in commercial nitrate of silver, 
but the presence of these salts in small quantity would have 
little effect except in reducing the strength of the baths. 
The peculiar photographic action of bad nitrate of silver is 
probably to be referred to a different cause, viz., to the pres- 
ence of oxidized organic matter. In the assay processes, frag- 
ments of charcoal are introduced to prevent the acid from 
bumping as it dissolves the silver. We have good reason for 
believing that during this process the nitric acid oxidizes the 
charcoal into a substance which has an affinity for the silver 
salt, and the author has found that nitrate of silver so pro- 
duced is altogether unfit for collodion photography. 

"When the old nitrate baths are precipitated by zinc, organic 
matter is carried down by the reduced silver, and the product, 



SILVER. 79 

if converted into nitrate of silver without previous fusion is 
useless for photographic purposes. 

Impurities of a similar kind, but in less quantity, have been 
detected in cases where no charcoal had been employed. 
Pieces of straw, etc., may perhaps fall into the acid; but, 
however this may be, the fact is certain that nitrate of silver 
prepared by dissolving silver in nitric acid, and evaporating 
to dryness without any crystallization, cannot be depended on 
for Photography. 

It has been suggested that the nitrate of silver for the bath 
should be made purposely by dissolving pure silver in pure 
nitric acid. Probably, however, no manufacturer would care 
to compete with the refiners, who are in a better position to 
supply the article at a reasonable price. Recrystallization 
seems, therefore, to be the proper remedy. The first crop of 
crystals may be dried off from the acid, and then crystallized a 
second time from distilled water ; after which the product 
will be in a pure state if charcoal and substances of that kind 
have been excluded. This second crystallization ought not, 
according to a competent authority, to add more than four- 
pence, or at most sixpence, to the price per ounce. 

A saturated solution of the purified crystals slowly restores 
the blue color of reddened litmus paper, if the nitric acid be 
expelled by heating to 240 deg. Fahr., previous to the second 
crystalization. This proceeding, however, is not actually neces- 
sary, inasmuch as a trace of adhering nitric acid can always 
be removed by carbonate of soda when making the bath ; and it 
is well-known that the presence of a little acid facilitates the 
crystallization of nitrate of silver. 

SILVER, NITKITE OF. 

Formula, AgN02,=154. 

Nitrite of silver is a compound of nitrous acid, or HNO^ 
with oxide of silver. It may be formed by heating nitrate of 
silver, so as to drive ofl: a portion of its oxygen, or by mixing 
nitrate of silver and nitrate of potash in equal parts, fusing 
strongly, and dissolving in a small quantity of boiling water : 
on cooling, the nitrite of silver crystallizes out, and may be 
purified by pressing blotting-pajDcr ; but the best method of 
producing it is to fuse pure nitrate of potash (free from chlo- 
ride) in a crucible, at a strong red heat, until a portion 
removed, dissolved in water and tested with nitrate of silver. 



ij0 OUTLINES OF GENERAL CHEMISTKY. 

is found to give a brownish precipitate due to a portion of 
free potasli ; when this is the case, it is allowed to cool, and 1 
part of the product is then dissolved in 20 parts of boiling 
water. Mix. Filter rapidly through a hot filter. On cooling, 
the nitrite of silver crystallizes out abundantly from the liquid. 
Properties. — Nitrite of silver is soluble in 120 parts of cold 
water, easily soluble in boiling water, and crystallizes on cool- 
ing, in long slender needles. A small quantity dissolved in 
the negative bath increases the sensitiveness and intensity of 
the collodion surface, but it has a tendency to produce 
fogging. 

SILYEE, OXIDE OF. 

Formula, Ag20=232. 

Oxide of silver may be obtained by adding a solution of 
potash (pure, or nearly free from chloride) to one of nitrate 
of silver ; a brown-black precipitate of oxide of silver falls, 
which may be collected on a filter and washed, or it may be 
allowed to settle down, the clear liquid poured off, fresh water 
added, and the oxide again allowed to settle ; this being 
repeated until the nitrate of potash and potash have been 
removed, which may be ascertained either by evaporating a 
little on a slip of glass, or, if excess of potash was added in 
the first place, by testing with red litmus paper. It may be 
kept in a bottle in a moist state, its use being to neutralize 
nitrate of silver baths, and to separate oxide of copper from 
nitrate of silver ; the bottle should be kept in the dark. 

SILVER, SULPHIDE OF. 

Formula, Ag2S=248. 

This compound, sometimes termed sulphuret of silver, is 
formed by the action of sulphur upon metallic silver, or of 
sulphuretted hydrogen or hydrosulphate of ammonia upon 
the silver salts: the decomposition of hyposulphite of silver 
also furnishes the black sulphide. 

Sulphide of silver is insoluble in water, and in those sub- 
stances which dissolve the chloride, bromide and iodide, such 
as ammonia, hyposulphites, cyanides, etc. ; but it dissolves in 
nitric acid, being converted into soluble sulphate and nitrate 
of silver. 



SODA. 81 

The color of precipitated sulphide of silver varies with the 
quantity present ; it is black when in mass, but yellowish- 
brown in a state of fine division. 



SODA, ACETATE OF. 
Formula, Na,C2H302 + 3H/)=136. 

An easily crystalHzable salt, employed commercially as a 
source of acetic acid. There are several qualities, some of 
which contain brown empyreumatic matter, but the recrystal- 
lized salt can usually be obtained pure. It is readily soluble in 
water, and also in alcohol. 

This acetate is undoubtedly the best form to employ in pho- 
tography, since acetate of potash is deliquescent, and cannot 
easily be kept dry. It is often used along with chloride of 
gold for toning silver prints. 

SODA, CAKBONATE OF. 

Formula, Na3,CC3 + 10Aq.==286. 

There are two carbonates of soda. The one (carbonate) is 
the common washing soda. The other is the bicarbonate, 
which is used in photography sometimes for neutralizing a too 
acid silver bath, and very often in the chloride of gold toning 
bath. 

SODA, CITRATE OF. 

Formula, Na3C6H507=258. 

This salt, the composition of which is analogous to the 
citrate of silver, q. -y., yields well-defined crystals, but as it is 
sometimes difficult to obtain it in commerce, instructions are 
given in this work for making it extemporaneously by neutral- 
izing 56 grains of pure citric acid with 'o^ grains of dry 
sesquicarbonate of soda, quantities which will yield 95 grains 
of citrate of soda. 

The crystals are very soluble in water, and produce a white 
flocculent deposit of citrate of silver in solution of nitrate of 
silver. 



82 OTJTLINKS OF GENERAL CHEMISTEY- 

SODA, HYPOSULPHITE OF, 

Formula, NajSjO +5Aq.=248, 

is a salt largely used in photography for fixing images. This 
it eflPects by virtue of the power which it possesses of dissolv- 
ing the haloid salts of silver. This property was discovered 
many years ago by Sir John Herschel. 

Hyposulphite of soda is made on an extensive scale, mainly 
for the purposes of the paper manufacturers, who employ it 
as an antichlor, or absorbent of the chlorine used in bleaching 
the pulp of linen and cotton rags. Some of the inferior 
qualities have a yellowish color, arising probably from sul- 
phide or free sulphur. This kind should not be used for a 
fixing solution, as it is sure to impregnate the print with that 
deadly enemy to silver photographs— sulphur. Acids, even 
in minute quantity, decompose it, liberating sulphur. Hence 
the importance of keeping hyposulphite solutions in a slightly 
alkaline, or at least neutral, state. 

SODA, PHOSPHATE OF. 
Formula, Na^HPOi+lBHsO^SSS. 

There are several phosphates of soda, but the common phos- 
phate has the above composition. It forms large transparent 
crystals which etfloresce, and become opaque on the exterior • 
soluble in four parts of cold water, the solution possessing a 
faint alkaline reaction ; precipitates nitrate of silver yellow. 

Pyrophosphate of soda, so called, is prepared by heating 
the ordinary phosphate until the whole of the water has been 
expelled ; when the residue is dissolved in water it is found to 
have been completely altered in properties, as it precipitates 
nitrate of silver white instead of j^ellow ; it crystallizes in 
prisms. Formula, 2Na,P03. 

SODIUM, AURO-CHLORIDE OF. 

Formula, NaCl,AuCl3 + 4:Aq.=39Y.6. 

This salt is a double chloride of gold and sodium, with four 
atoms of water of crysallization. It has been proposed as a 
convenient substitute for the ordinary chloride of gold, as it is 
not deliquescent, and forms long orange-yellow, four-sided 



SODA. 83 

prisms. Another advantage is the absence of free acid in the 
double salt, w'hereas the ordinary chloride of gold always con- 
tains hydrochloric acid. The auro-chloride, however, will not 
in all probability come into general use in photography, since 
it is evidently open to adulteration, and although theoretically 
neutral, yet the manufacturers themselves allow that it will 
not be safe to calculate upon perfect neutrality, since traces of 
hydrochloric acid are apt to remain in the interstices of the 
crystals. 

It is prepared by evaporating to dryness a solution of four 
parts of gold in aqua regia, dissolving the residue in water, 
adding one part of common salt, evaporating the whole down 
to four parts, and leaving it to crystallize by cooling. 

The pure salt contains 49. Y per cent, of gold. The amount 
of gold may be ascertained by taking a given weight of the 
compound, dissolving it in water, and boiling it with half its 
weight of oxalic acid ; the whole of the gold separates in the 
metallic state, and may then be washed, dried and weighed. 
The gold may also be separated by the addition of five times 
the weight of sulphate of iron, according to directions under 
Gold, Chlokide of. 



SODIUM, CHLORIDE OF. 

Formula, NaCl^SS.S. 

Common salt exists abundantly in Nature, both in the form 
of solid rock-salt, and dissolved in the waters of the ocean. 

Properties. — Fusible without decomposition at low redness, 
but sublimes at higher temperatures ; the melted salt con- 
cretes into a hard wliite mass on cooling. Nearly insoluble in 
absolute alcohol, but dissolves in minute quantity in rectified 
spirit. Soluble in three parts of water, both hot and cold. 
Crystallizes in cubes, which are anhydrous. 

Impurities of Common Salt. — Table salt usually contains 
some chloride of magnesium, which, being deliquescent, pro- 
duces a dampness by absorption of atmospheric moisture : 
Sulphate of soda is also commonly present. The salt may be 
purified by repeated recrystallization, but it is more simple to 
prepare the pure compound directly by neutralizing hydro- 
chloric acid with carbonate of soda. 



84 . OUTLINES OF GENERAL CHEMISTBY. 

SODA, SULPHATE OF. 
Forimila, Na2S03—126. 

This salt, which has recently been introduced into the pho- 
tographic laboratory for the special purpose of preserving 
aqueous solutions of pyrogallol, is formed by passing a stream 
of sulphurous acid gas through a concentrated solution of 
carbonate of soda to saturation. It forms large regular crys- 
tals, which are soluble in three parts of water at the normal 
temperature. The soluble metallic sulphites possess powerful 
bleaching properties, and their strong affinity for oxygen, by 
which they are converted into sulphates, renders them valua- 
ble for the prevention of oxidation of such substances as 
pyrogallol. 

SULPHOCYANIDES. 

Solutions of the sulphocyanides of ammonium or potassium 
have been recommended for fixing positive photographs, in- 
stead of hyposulphite of soda. But it is doubtful whether any 
advantage is gained. 

SULPHURETTED HYDROGEN. 

See Hydrosulphuric Acid. 

SULPHURIC ACID. 
Formula, H3SO,— 98. 

Sulphuric acid is the highest oxide of sulphur ; the liquid 
acid, when most concentrated, always has one atom of water, H^ 
0,S03 or H2SO4. The commercial process for the manufacture 
of sulphuric acid is exceedingly ingenious and beautiful, but it 
involves many complicated reactions. The sulphur is first burnt 
into gaseous sulphurous acid, SO2, and then, in the presence 
of vapor of water, by the agency of binoxide of nitrogen gas 
an additional atom of Oxygen is conveyed to the SOj, convert 
ing it into H2SO1, or sulphuric acid. 

Properties. — Anhydrous sulphuric acid is a white crystal- 
line solid. The strongest liquid acid always contains one atom 
of water, which is chemically combined with it, and cannot be 
driven off by the application of heat. 



SULPHURIC ACID. 85 

This monohydrated sulphuric acid, represented by the for- 
mula H2SG4, is a dense fluid, having a specific gravity of about 
1.845 ; boils at 620 deg. Fahr., and distills without decomposition. 
It is not volatile at common temperatures, and therefore does not 
fume like nitric or hydrochloric acid. The concentrated acid 
may be cooled down, even to zero, without solidifying ; but a 
weaker compound, containing twice the quantity of water, and 
termed glacial sulphuric acid, crystallizes at 40 deg. Fahr. Sul- 
phuric acid is intensely acid and caustic, but it does not destroy 
the skin or dissolve metals so readily as nitric acid. It has an 
energetic attraction for water, and when the two are mixed, 
condensation ensues, and much heat is evolved ; four parts of 
acid and one of water produce a temperature equal to that of 
boiling water. Mixed with nitric acid and a certain propor- 
tion of water, it forms the liquid called by photographers 
nitro-sulphuric acid. 

Sulphuric acid possesses intense chemical powers, and dis- 
places the greater number of ordinary acids from their salts. 
It chars organic substances, by removing the elements of water, 
and converts alcohol into ether in a similar manner. The 
strength of a given sample of sulphuric acid may be calculated, 
nearly from its specilic gravity, and a table is given by Dr. 
Ure for that purpose. (See Appendix.) It is quite necessary, 
however, that the temperature should be attended to in taking 
the specific gravity, since a few degrees higher or lower than 
the point mentioned causes considerable difference. 

Impurities of Commercial Suljjhuric Acid. — The liquid acid 
sold as oil of vitriol is tolerably constant in composition, 
and seems to be as well adapted for photographic use as the 
pure sulphuric acid, which is far more expensive. The specific 
gravity is often about 1.836 at 60 deg. Fahr, but an acid of 1.843 
or 1.845 at 60 deg. Fahr., can always be obtained of the manu- 
facturers, and is preferable for employment in photography. 
11 a drop, evaporated upon platinum-foil, gives a fixed residue, 
probably bisulphate of potash is present. A milkiness, on 
dilution, indicates sulphate of lead ; a little sulphate of lead how- 
ever, would not interfere, and may be disregarded. Commer- 
cial sulphuric acid sometimes bleaches sulphate of indigo, on 
boiling a diluted solution ; this is due to traces of nitric acid 
present in the oil of viriol. 

Test for Sulphuric Acid. — If the presence of sulphuric acid, 
or a soluble sulphate, be suspected in any liquid, it may be 
tested for by adding a few drops of dilute solution of chloride 
of barium, or of nitrate of baryta. A white precipitate, in- 



86 OUTLINES OF GENERAL CHEMISTRY. 

soluble in nitric acid, indicates sulphuric acid. If the liquid 
to be tested is very acid, from nitric or hydrochloric acid, it 
must be largely diluted before testing, or a crystalline precipi- 
tate will form, caused by the sparing solubility of the chloride 
of barium itself in acid solutions. 

TANNm. 
Formala, Ci4HioOs=322. 

Various organic substances, possessing an astringent action, 
have been termed " Tannin ;" such, for instance, as the extractive 
matters from bark, used in tanning hides, the astringent prin- 
ciples of tea and coffee, etc. The most important, however, 
is the tannin of the gall nut, known as " tannic acid," or " gal- 
lo-tannic acid." 

Gall nuts contain as much as two-thirds of their weight of 
tannic acid, which is extracted by reducing the nuts to powder 
and digesting them with washed ether : the decanted liquid se- 
parates on standing into two portions, the lower being an aque- 
ous solution of tannic acid, the watery constituent of which is 
derived from the washed ether [see Ether). On evaporating 
the aqueous solution to dryness, a poros, buff-colored residue 
of amorphous tannic acid is obtained. 

Tannic acid is freely soluble in water, but it rarely yields a 
clear solution, on account of traces of remaining resinous mat- 
ter. The reaction to test-paper is slightly acid, and on adding 
an alkal a "tannate" is formed, but the alkaline tannates are very 
unstable, and tend to absorb oxygen and become brown. Tan- 
nic acid gives with the persalts of iron a violet-black precipi- 
tate, which is the basis of common writing ink. Solutions of 
gelatine are precipitated by tannic acid in the form of tan no- 
gelatine, the material of leather; albumen is also coagulated 
by tannin. When tannic acid is heated to about 600 deg. Fahr., 
it is decomposed, and yields pyrogallic acid. Nitrate of silver 
is not precipitated by tannic acid, but suffers a slow reduction 
to the state of metallic silver. 

TARTARIC ACID. 

Formula, CiH6O6=150. 

This acid is derived from a substance called tartar, de- 
posited from the juice of the grape during fermentation. This 
tartar is an acid tartrate of potash, and purified by crystalliza- 



TETEATHIONIC ACID. 87 

tion and deprived of its base it furnishes this acid. Tartaric 
acid forms colorless, transparent, rhombic prisms, very soluble 
in water and alcohol ; it is a bibasic acid. It is used for most 
purposes in photography for which citric acid is employed. 

TETEATHIONIC ACID. 

Formula, S,O6=104. 

This acid is one of the oxides of sulphur, belonging to the 
interesting series designated by Berzelius the " polythionic 
acids." The composition of this series may be represented as 
follows : — 

Dithionic acid HgSgOj. 

Trithionic acid HaSsOg. 

Tetrathionic acid HaS^Oe. 

Pentathionic acid HaSsOg. 

The amount of oxygen in all is the same, that of the other ele- 
ment increases progressively ; hence the highest membei- tf ih j 
series might by losing sulpur descend gradually until itr? idies 
the condition of the lowest. 

Such a transition is not only theoretically possible, but ti.i-re 
is an actual tendency to it, all the acids being unstable with 
the exception of the dithionic. The alkaline salts of these 
acids are more unstable than the acids themselves ; a solution 
of tetrathionate of soda becomes milky in the course of a few 
days from deposition of sulphur, and, if tested, is then found 
to contain ^rithionate and eventually ^^thionate of soda. 

The presence of hyposulphite of soda increases the instabil- 
ity of tetrathionate of soda. A solution of the latter may be 
preserved for many hours unchanged, but if a few crystals of 
hyposulphite of soda be dropped in, it begins very shortly to 
deposit sulphur, and continues to do so for several days. At 
the same time the liquid acquires an acid reaction to test- 
paper, and produces effervescence on the addition of carbonate 
of lime. 

Tetrathionate of soda may be formed by acting on hypo- 
sulphite of soda with iodine ; a colorless solution is obtained, 
which, in addition to the new salt, contains iodide of sodium. 
Perchloride of iron, chloride of copper, and chloride of gold, 
decompose hyposulphite of soda, and form tetrathionate 
among other products. Acids in a free state have often the 
same effect, since they liberate sulphurous acid, which, in con- 



88 OUTLINES OF GENERAL CHEMISTRY. 

tact with hypoSSilpliite of soda, forms trithionate and te- 
trathionate of soda. 

• TURMERIC. 

Turmeric is the rhizome of an Indian plant. It possesses a 
yellow coloring matter, curcumin, soluble in ether and alcohol. 
Paper brushed with a strong decoction of turmeric is used by 
the chemist as a test for alkalinity ; free alkalies or alkaline 
bases change the color from yellow to brown. It is not so 
sensitive as reddened litmus, but is useful as indicating a 
strong alkaline reaction. , 

URAKIUM, NITRATE OF. 

(UrO,),(N03>-.6Aq.=384. 

This salt is obtained by dissolving one of the higher oxides 
of uranium in nitric acid, and evaporating over a water bath. 

The remarkable photographic properties of some of the 
uranium salts were tirst discovered by Mr. Burnett, of Edin- 
burgh. Since then the nitrate of uranium has been used 
conjoined with silver or gold in several processes. 

The rationale seems to be this. The uranium per-salt is 
reduced by light to a proio-salt, which, when the exposed 
paper or film is brought in contact with gold or silver solutions, 
reduces them by again absorbing oxygen and passing into a 
per-salt. 

WATER. 
Formula, H20=18. 

Water is an oxide of hydrogen, containing single atoms of 
each of the gases. 

Distilled water is water which has been vaporized and again 
condensed ; by this means it is freed from earthy and saline 
impurities, which, not being volatile, are left in the body of 
the retort. Pure distilled water leaves no residue on evapor- 
ation, and should remain perfectly clear on the addition of 
nitrate of silver, even when exposed to the light ; it should 
also be neutral to test-paper. 

The condensed water of steam-boilers, often sold in pro- 
vincial towns and elsewhere as distilled water, is apt to be con- 
taminated with oily and empyreumatic matter, which discoloas 



WATER. 89 

nitrate of silver, and is therefore injurious. Newly-made 
metal stills also fail at first in giving a pure product, from the 
presence of grease or dirt in the soldered joints. Professor 
Tomlinson has shown that a very minute amount of grease on 
water may be detected by camphor, which, thrown on 
water free from grease, rapidly spins round, until it has dis- 
solved or has evaporated, but this motion is prevented by a 
trace of fatty matter. 

Rain water, having undergone a natural process of distilla- 
tion, is free from inorganic salts, but it usually contains a 
minute portion of ammonia, which gives it an alkaline reac- 
tion to test-paper. It is very good for photographic purposes, 
if collected in clean vessels, but when taken from a common 
rain-water tank should always be examined, and if much 
organic matter be present, tinging it of a brown color, and 
gradually reducing nitrate of silver in presence of light, it 
must be rejected. 

Spring or river water, commonly known as "hard water," 
usually contains sulphate of lime, and carbonate of lime dis- 
solved in carbonic acid ; also chloride of sodium in greater or 
less quantity. On boiling the water for twenty minutes or 
half an hour, the carbonic acid gas is evolved, and the greater 
part of the carbonate of lime (if any be present) deposits, 
forming an incrustation, which dissolved in acetic acid with 
effervescence. Spring water is less likely to contain brown 
organic matter than rain water. 

In testing water for carbonates, sulphates and chlorides, 
divide it into two parts, and add to the first a dilute solution 
of chloride of barium, and to the second nitrate of silver* — a 
milkiness which indicates the presence of either carbonate or 
sulphate in the first case, or of carbonate or chloride in the 
second. Next, acidify the two liquids with a few drops of 
glacial acetic acid. If the opalescence disappears and the 
fluid becomes clear, carbonates are present ; but if, as is more 
frequently the case, the cloudiness is only partially removed 
by the acetic acid, then the carbonate is mixed with sulphate 
or chloride, as the case may be. 

Water for the Nitrate Bath.—Qctxm^on hard water can 
often be used for making a nitrate bath when nothing better 
is at hand. The chlorides it contains are precipitated by the 
nitrate of silver, leaving small quantities of soluble NUrates 

*Tlie photographic nitrate bath cannot be used as a test, since the iodide 
of silver it contains is precipitated on dilution, giving a milkiness which 
might be mistaken for chloride of silver. 



90 OUTLINES OF GENERAL CHEMISTKY. 

in solution, which are not injurious. Carbonate of lime, if 
present, neutralizes acid, and may render the bath alkaline if 
salts of ammonia are present. Sulphate of lime causes no 
precipitation, and onght not theoretically to produce any 
injurious effect. It has, however, been spoken against by 
some, but whether from practical experience or not, the writer 
is uninformed. Organic matter will almost certainly be 
injurious to the bath, and, therefore, unless the purity of the 
rain water can be guaranteed, spring water will be preferable. 
Water for the Developing Solutions. — Carbonate of lime in 
a water ought not to unlit it for the developing liquid in pres- 
ence of free acetic acid, because the chalk would be decom- 
posed under such circumstances, and converted into acetate of 
lime, which would probably assist a little in increasing the 
inteiisity. In the case of sulphate of iron, chalky water 
throws down a small quantity of oxide of iron, which produces 
turpidiry and a rusty color at the edges of the pictures ; but 
any acid added to the developer dissolves it, and I'enders the 
liquid again clear. Chalk in water used for the gallic acid 
develo})er, makes it discolor quickly with nitrate of silver, 
but a mniim of acetic acid to each two ounces is a remedy, as 
before shown. 

Soluble chlorides are always injurious in water used for 
developing, because they decompose the nitrate of silver on 
the film, producing a curdy precipitate, and lessening the 
available strength. Acids have no effect in removing chlo- 
rides, but they may be separated by shaking up the water 
with a graduated quantity of nitrate of silver, and filtering it. 
In this process, however, there is always a difficulty in know- 
ing how much of the nitrate will be needed, since the least 
excess over the quantity required to decompose the salt causes 
the water to blacken on adding pyrogallic acid. This might 
be obviated, however, by keeping the pyrogallic acid dis- 
solved in glacial acetic acid, and adding the water only when 
required. Bear in mind also that it will not be necessary to 
remove every trace of salt from the water, since it is quite 
possible to bring out a perfect image with a developing solu- 
tion which produces a decided turbidity on first touching the 
film ; especially so if a few drops of nitrate of silver be added 
to the developer immediately before use. 



NITRATE OF SILVER. 91 



CHAPTER III. 



THE SALTS OF SILVER EMPLOYED IN PHOTOGRAPHY. 

By the term Salt of Silver we understand that the compound 
in question contains silver, but not in its elementary form ; 
the metal is in fact in a state of chemical union with other 
elements which diso;uise its physical properties, so that the salt 
possesses none of the external characters of the silver from 
which it was produced. 

The contents of this chapter may be arranged in three sec- 
tions ; the first describing the chemistry of the salts of 
silver ; the second, the action of light upon them ; the third, 
the preparation of a sensitive surface, with experiments illus- 
trating the formation of the photographic image. 

Section I. 

Chemistry of the Salts of Silver, 

The principal salts of silver employed in the photographic 
process is four in number, viz., nitrate of silver, chloride of 
silver, iodide of silver, and bromide of silver. In addition 
to these it will be necessary to describe the oxides of silver. 

THE PREPARATION AND PROPERTIES OF THE NITRATE OF SILVER. 

Nitrate of silver is prepared by dissolving metallic silver in 
nitric acid. Nitric acid is a powerfully acid and corrosive 
substance, containing two elementary bodies united in definite 
proportions. These are nitrogen and oxygen ; the latter being 
present in greatest quantity. 

Nitric acid is a powerful solvent for the metallic bodies gen- 
erally. To illustrate its action in that particular, as contrasted 
with other acids, place pieces of silver-foil in two test-tubes, 
the one containing dilute sulphuric, the other dilute nitric acid ; 
on the application of heat a violent action soon commences in 
the latter, but the former is unaffected. In order to understand 
this, it must be borne in mind that when a metallic substance 
dissolves in an acid, the nature of the solution is different from 



92 OUTLINES OF GENERAL CHEMISTKY. 

that of an aqueous solution of salt or sugar. If salt water be 
boiled down until the whole of the water has evaporated, the 
salt is recovered with properties the same as at first ; but if a 
similar experiment be made with a solution of silver in nitric 
acid, the result is different : in that case we do not obtain me- 
tallic silver on evaporation, but silver combined with oxygen 
and nitric acid, both of which are in a state of chemical com- 
bination with the metal. 

Properties of Nitrate of Silver. — In preparing nitrate of sil- 
ver, when the metal is dissolved, the solution is boiled down 
and set aside to crystallize. The salt, however, so obtained is 
still acid to test-paper, and requires either recrystallization, or 
careful heating to about 300 deg. Fahr. It is this retention of 
small quantities of nitric acid, and other impurities, which 
renders much of the commercial nitrate of silver useless for 
photography, until after a second crystallization. 

Pure nitrate of silver occurs in the form of white crystalline 
plates, which are very heavy, and dissolve readily in an equal 
weight of cold water. The solubility is much lessened by the 
presence of free nitric acid, and in the concentrated nitric acid 
the crystals are almost insoluble. Boiling alcohol takes up 
about one-fourth part of its weight of the crystallized nitrate, 
but deposits nearly the whole on cooling. Nitrate of silver 
has an intensely bitter and nauseous taste ; acting as a caustic,, 
and corroding the skin by prolonged application. Its aqueous 
solution does not redden blue litmus-paper; on the contrary, 
the pure recrystallized and dried nitrate of silver restores the 
blue color of paper previously reddened. 

Heated in a crucible the salt melts, and when poured into a 
mould and solidified, forms the white lunar caustic of com- 
merce. At a still higher temperature it is decomposed, and 
bubbles of oxygen gas are evolved ; the melted mass cooled 
and dissolved in water yields a solution, which contains nitrite 
in addition to nitrate of silver.* 

THE CHEMISTRY OF THE CHLORIDES OF SILVER. 

Preparation of Chloride of Silver (AgCl). — The ordinary 
white chloride of silver may be prepared in two ways — by the 
direct action of chlorine upon metallic silver, and by double 
decomposition between two salts. 

* Nitrate of silver differs from the nitrate in containing less oxygen, and 
is formed from it by the abstraction of two atoms of that element ; it is 
described in the vocabulary. 



CHLORIDES OF SILVER, 93 

If a plate of yjolished silver be exposed 'to a current of 
chlorine gas, it becomes, after a short time, coated on the sur- 
face with a superficial film of white powder. This powder is 
chloride of silver, containing the two elements, chlorine and 
silver, united in single equivalents. 

Preparation of Chloride of Silver hy Double Deoomposi' 
tion. — In order to illustrate this, take a solution in water of 
chloride of sodium, or " common salt," and mix it with a solu- 
tion containing nitrate of silver ; immediately a dense, curdy, 
white precipitate falls, which is the substance in question. 

In this reaction the elements change places ; the chlorine 
leaves the sodium M'ith which it was previously combined, and 
crosses over to the silver ; the nitric acid is released from the 
silver, and unites with the sodium ; thus 

mCl+Ag,N03=Na,N03+AgCl. 

This interchange of elements is an example of double de- 
composition. 

In preparing chloride of silver by double decomposition, the 
white clotty masses which first form must be washed re- 
peatedly with water, in order to free tlieni from soluble nitrate 
of soda, the other product of the change. When this is done, 
the salt is in a pure state, and ma_y be dried, etc., in the usual 
way. 

Properties of Chloride of Silver. — Chloride of silver forms 
a soft white powder resembling common chalk or whiting. It 
is tasteless and insoluble in water ; unaffected by boiling with 
the strongest nitric acid, but sparingly dissolved by concen- 
trated hydrochloric acid. 

Ammonia dissolves chloride of silver freely, as do solutions 
of hyposulphite of soda and cyanide of potassium. Concen- 
trated solutions of the sulphocyanide, chloride, iodide, and 
bromide of potassium, sodium, and ammonium, are likewise 
solvents of chloride of silver, but to a limited extent. 

Dry chloride of silver carefully heated to redness fuses, and 
concretes on cooling into a tough and semi-transparent sub- 
stance, which has been termed horn silver or luna cornea. 

Placed in contact with metallic zinc or iron acidified with 
dilute sulphuric acid, chloride of silver is reduced to the 
metallic state, the chlorine passing to the other metal under 
the decomposing influence of the galvanic current which is 
established. 

2AgCl + Zn=ZnCl2+2Ag. 



94 OUTLINES OF GENERAL CHEMISTRY. 

Preparation and Properties of Suhchloride of Silver. — If 
a plate of polished silver be dipped into a solution of per- 
chloride of iron, a black stain is produced, the perchloride 
sinking to the state of a proto-chloride of iron, and losing a por- 
tion of chlorine, which passes to the silver and converts it 
superficially into a suhchloride of silver. 

This compound differs from the white chloride of silver in 
containing less chlorine. As it has not been suflScientlj pure 
for analysis, no formula can be assigned to it. The only facts 
certainly ascertained with regard to the suhchloride of silver 
are, that it is a pulverulent substance of a bluish-black color, 
not easily affected by nitric acid, but decomposed by fixing 
agents, such as ammonia, hyposulphite of soda, or cyanide of 
potassium into chloride of silver, which dissolves, and insoluble 
metallic silver. 

THE CHEMISTRY OF IODIDE OF SILVER. 

The properties of iodine are described in the first part of 
the work ; they are analogous to those of chlorine and bromine, 
the silver salts formed by these elements bearing also a strong 
resemblance to each other. 

Preparation and Properties of Iodide of Silver (Agl). — 
Iodide of silver may be formed in an analogous manner to the 
chloride, viz., by the direct action of the vapor of iodine upon 
metallic silver, or by double decomposition between solutions 
of iodide of potassium and nitrate of silver. 

When prepared by the latter mode it forms an impalpable 
powder, the color of which varies with the manner of precipi- 
tation. If the iodide of potassium be in excess, the iodide of 
silver, nearly white, falls to the bottom of the vessel ; but 
with an excess of nitrate of silver it is of a straw-yellow tint. 
This point may be noticed, because the yellow condition is the 
one adapted for photographic use, the other being insensible to 
the influence of light. 

Iodide of silver is tasteless and inodorous ; insoluble in water 
and in dilute nitric acid. It is scarcely dissolved by ammonia, 
which serves to distinguish it from the chloride of silver, freely 
soluble in that liquid. Hyposulphite of soda and cyanide of 
potassium both dissolve iodide of silver ; it is also soluble in 
solutions of sulphocyanide, bromide, and iodide of potassium, 
sodium and ammonium, as will be further explained in Chap- 
ter Y. 

Iodide of silver is reduced by metallic zinc in the same 



OXIDES OF SILVER. 95 

manner as the chloride of silver, forming soluble iodide of zinc 
and leaving a black powder of metallic silver. 

2 Agl + Zn=Znl2 + 2 Ag. 



THE PBEPAEATION AND PROPERTIES OF BROMIDE OF SILVER. 

This substance so closely resembles the corresponding salts 
containing chlorine and iodine, that a short notice of it will 
suffice. 

Bromide of silver (AgBr) is prepared by exposing a sil- 
vered plate to the vapor of bromine, or by adding solution of 
bromide of potassium to nitrate of silver. It is insoluble in 
water, slightly yellow in color, and distinguished from iodide 
of silver by dissolving in strong ammonia and in chloride of 
ammonium. It is freely soluble in hyposulphite of soda and 
in cyanide of potassium. 

CHEMISTRY OF THE OXIDES OF SILVER. 

The Protoxide of Silver (AgjO). — If a little potash or 
ammonia be added to a solution of nitrate of silver, an 
olive-brown substance is formed, which, on standing, collects 
at the bottom of the vessel. This is oxide of silver, displaced 
from its previous state of combination with nitric acid by the 
stronger oxide, potash. Oxide of silver is soluble to a very 
minute extent in pure water, the solution possessing an alka- 
line reaction to litmus ; it is easily dissolved by nitric or acetic 
acid, forming a neutral nitrate or acetate of silver ; it also 
dissolves in ammonia (ammonia-nitrate of silver), nitrate of 
ammonia, hyposulphite of soda, and cyanide of potassium. 

The Suboxide of Silver {A.gjd '() . — This compound is dis- 
tinguished from the protoxide by the action of hydrochloric 
acid, which, with the former, produces the white chloride, but 
with the latter the day^h sub-chloride. It has probably never 
been obtained pure, but a certain amount is obtained, mixed, 
however, with silver and protoxide, on boiling arsenite of sil- 
ver, 3Ag,As03, with a solution of potash : a black powder 
results, which is the above mixture. On treating this powder, 
after washing, with hydrochloric acid, a mixture of chloride, 
subchloride and metallic silver is produced, from which, after 
washing, the latter may be dissolved out by nitric acid, leaving 
the dark mixture of chlorides. 



96 OUTLINES OF GENERAL CHEMISTRY. 

Section II. 
On the Photographic Properties of the Salts of Silver. 

In addition to tlie salts of silver described in the first sec- 
tion of this chapter, there are many others well known to 
chemists, as the acetate of silver, the snljDhate, the citrate of 
silver, etc. Some occnr in crystals which are soluble in water, 
while others are pulverulent and insoluble. 

The salts of silver formed by colorless acids are white when 
first j)repared, and remain so if kept in a dark place ; but they 
possess the remarkable peculiarity of being darkened by ex- 
posure to light, either alone or in contact with organic sub- 
stances. 

Action of Light upon the Nitrate of Silver. — The nitrate 
of silver is one of the most permanent of silver salts. It 
may be preserved unchanged in the crystalline form, or in 
solution in distilled water for an indefinite length of time, 
even when constantly exposed to the light of day. 

I^itrate of silver may, however, be rendered susceptible to 
the influence of light by adding to its solution or gariic matter, 
vegetable or animal. The phenomena produced in this case 
are well illustrated by dipping a sheet of white paper in a 
solution of nitrate of silver, and exposing it to the direct rays 
of the sun ; it slowly darkens, until it becomes nearly black. 
The stains upon the skin produced by handling nitrate of sil- 
ver are caused in the same way, and are seen most evidently 
when the part has been exposed to light. 

The varieties of organic matter which especially facilitate 
the blackening of nitrate of silver are such as tend to ahso7'l) 
oxygen ; hence pure vegetable fibre, free frem chlorides, such, 
for instance, as the Swedish filtering paper, is not rendered 
very sensitive by being simply brushed with solution of the 
nitrate, but a little grape sugar added soon determines the 
decomposition, 

Pecom.position of Chloride, Bromide and Iodide of Silver 
hy Light. — Pure chloride of silver, prepared in the moist way, 
changes slowly from white to violet on exposure to light. 
Bromide of silver becomes of a gray color, but is less effected 
than the chloride. Iodide of silver (if free from excess of 
nitrate of silver) does not alter in appearance by exposure 
even to the sun's rays, but retains its yellow tint unchanged. 
Of these three compounds, therefore, chloride of silver is the 
most visibly acted on by light, and papers prepared with this 



ACTION OF LIGHT ON CHLORIDE OF SILVER, 97 

salt will become far darker on exposure than others coated 
with bromide or iodide of silver. 

There are certain conditions which render the action of 
light upon the chloride of silver more decided. These are, 
first, an excess of soluble salt of silver, such as the nitrate, 
and second, the presence of organic matter. Pure chloride of 
silver would be useless as a photographic agent, but a chloride 
with excess of nitrate takes a strong impression. Even iodide 
of silver, ordinarily unaffected, is blackened by light when 
moistened with a solution of the nitrate of silver. 

Organic matter combined with chloride and nitrate of silver 
gives a still higher degree of darkening in the solar ray, and 
in this way the photographic papers are prepared. 

Action of Light xijpon Organic Compounds of Silver. — On 
adding diluted albumen, or white of egg, to solution of nitrate 
of silver, a flocculent deposit forms, which is a compound of 
the animal matter with oxide of silver, and is known as 
" albuminate of silver." This substance is at first white, but 
on exposure to light it assumes a brick-red color. 

Caseine, the animal principle of milk, is coagulated by 
nitrate of silver, the product behaving in the same manner as 
the albuminate when exposed to light. G-elatine does not 
precipitate nitrate of silver, but if a sheet of transparent gela- 
tine be allowed to imbibe a solution of the nitrate, it combines 
with it, and the product becomes of a clear ruby-red tint on 
exposure to light. 

Many other organic com]30uiids of silver are darkened by 
light. The white citrate of silver changes to a red color. 
Glycyrrhizine, the sugar of liquorice, also forms a white com- 
pound with oxide of silver, which becomes brown or red in 
the sun's rays. 

The photographic use of the organic salts of silver, and the 
extent to which they are affected by light, will be further 
considered in the eighth chapter, when speaking of the theory 
of positive printing. 

SIMPLE EXPERIMENTS ILLUSTRATING THE ACTION OF LIGHT UPON 
A SENSITIVE LAYER OF CHLORIDE OF SILVER ON PAPER. 

In the performance of the most simple experiments on the 
decomposition of silver salts by light, the student may employ 
ordinary test-tubes, in which small quantities of the two 
liquids required for the double decomposition may be mixed 
together. 



98 THEORY OF PH<JTOGRAPHY, 

When, however, concentrated solutions are used in this way, 
the insoluble silver salt falls in dense and clotted masses, which, 
exposed to the sun's rays, quickly blacken on the exterior, but 
the inside is protected, and remains white. It is of importance, 
therefore, in photography that the sensitive material should 
exist in the form of a surface, in order that the various particles 
of which it is composed may each one individually be brought 
into relation with the disturbing force. 

Full directions for the preparation of sensative photographic 
paper are given in another portion of this work. The follow- 
ing is the theory of the process : A sheet of paper is treated 
with solution of chloride of sodium or ammonium, and sub- 
sequently with nitrate of silver ; hence results a formation of 
chloride of silver in a line state of division, with an excess of 
nitrate of silver, sirice the silver bath is applied after the salt- 
ing solution, and is made purposely of a greater strength. 

Illustrative Experiment No. I. — Place a square of sensitive 
paper, prepared as above, in the direct rays of the sun, and 
observe the gradual process of darkening which takes place ; 
the surface passes through a variety of changes in color until it 
becomes of a deep chocolate brown. If the light is tolerably 
intense, the brown shades are probably reached in from three 
to five minutes ; but the sensibility of the paper, and also the 
nature of the tints, will vary much with the character of the 
organic matter present. 

Experiment Wo. IT. — Lay a device cut from black paper 
upon a sheet of sensitive paper, and compress the two together 
by means of a sheet of glass. After a proper length of expos- 
ure the figure will be exactly copied with the tint reversed ; the 
black paper, protecting the sensitive chloride beneath, pro- 
duces a white figure upon a dark ground. 

Experiment No. III. — Repeat the last experiment, substi- 
tuting a piece of lace or gauze-wire for the paper device. Thi& 
is intended to show the minuteness with which objects can be 
copied, since the smallest filament will be distinctly represented. 

Experiment No. IV. — Take an engraving in which the con- 
trast of light and shade is tolerably well marked, and having 
laid it closely in contact with the sensitive paper, expose as 
before. This experiment shows that the surface darkens in 
degrees proportionate to the intensity of the light, so that the 
half shadows of the engraving are accurately maintained, and 
a pleasing gradation of tone is produced. 

In the darkening of photographic papers, the action of the 
light is quite superficial, and although the black color may be 



CHEMICAL CHANGES PRODUCED BY LIGHT. 99 

intense, yet the amount of reduced silver which forms it is so 
small that it cannot be conveniently estimated by chemical 
reagents. This is well shown by the results of an analysis per- 
formed by the author, in which the total weight of silver 
obtained from the blackened sheet, measuring nearly 24 by 18 
inches, amounted to less than half a grain, it becomes, there- 
fore, of great importance in preparing sensitive paper to 
attend to the condition of the surface layer of particles, the 
action rarely extending to those beneath. The use of albu- 
men, gelatine, etc., which will be explained subsequently, has 
reference to this among other advantages, and secures a better 
and more sharply defined print. 

THE NATURE OF THE CHEMICAL CHANGES PRODUCED BY LIGHT 
UPON COMPOUNDS OF SILVER. 

Action of Light iipon CJiloride of Silver. — This may be 
studied by suspending pure chloride of silver in distilled 
water, and exposing it to the sun's rays for several days. 
When the process of darkening has proceeded to some extent, 
the supernatant liquid will be found to contain free chlorine. 

The luminous rays appear to loosen the affinity of the ele- 
ments chlorine and silver for each other, hence a portion of 
chlorine is separated, and the white protochloride is converted 
into a violet subchloride of silver. If an atom of nitrate of 
silver be present, the liberated chlorine unites with it, dis- 
placing nitric acid, and forming again chloride of silver, which 
is decomposed in its turn. The excess of nitrate of silver thus 
assists the darkening of chloride of silver by rendering the 
chain of chemical affinity more complete, and preventing an 
accumulation of chlorine in the liquid. 

The violet-colored compound, the product of the darkening 
action of light, has been spoken of by some writers as a mix- 
ture of chloride of silver and metallic silver, but the fact that 
white chloride of silver will darken in the sun's rays, even 
when covered with strong nitric acid, proves that the color is 
not due to metallic silver, but to a subchloride. 

The properties of this subchloride are as follows : It is very 
little affected by nitric acid, but quickly acted on by those 
bodies which photographers employ as fixing agents, viz., 
ammonia, hyposulpliite of soda, and cyanide of potassium, the 
greater part being dissolved by the fixing agent, and a minute 
portion of metallic silver remaining insoluble. 

Chemical Changes in Organic Salts of Silver. — White albu- 
minate of silver is soluble in ammonia, and also in hyposul- 



100 THEORY OF PHOTOGRAPHY, 

phite of soda ; but after having been reddened by exposure to 
light, very little effect is produced upon it by these fixing 
agents. Gelatine, saturated with nitrate of silver and exposed 
to light, loses its characteristic property of dissolving iu hot 
water, but when treated with boiling solution of potash it is 
taken up, forming a clear liquid of a blood-red color. 

From these facts, and others not mentioned, we infer that 
in the action of light upon organic salts of silver, the oxide of 
silver loses oxygen, and sinks to the state of a suboxide, just 
as chloride of silver parts with a portion of chlorine when 
exposed to light, and becomes a subchloride. The organic 
body is probably oxidized, and the |)roducts are left in com- 
bination with each other. The present state of our knowledge 
will not allow of more than a general indication of the nature 
of these changes ; no exact formulas can be given. 

Nature of the Blackening of Photographic Papers. — In 
this case both chloride of silver and nitrate of silver are pres- 
ent, supported by an organic basis. To ascertain the nature 
of the darkening action of light upon a surface of this kind, it is 
better to spread the chloride upon a glass plate, and to make two 
experiments, adding organic matter in one, and omitting it in 
the other. Chloride of silver does not readily adhere to glass 
unless the surface of the glass be finely ground, and even then 
very careful manipulation is required. A more simple plan is to 
employ collodion as a vehicle for the chloride, and this we may 
do with impunity, since it is known that pyroxyline, the basis 
of collodion, has comparatively little effect upon the salts of 
silver, and behaves towards them almost in the manner of an 
inert substance, such us glass or porcelain. Having therefore 
taken a samj^le of ordinary collodion, dissolve in each ounce 
seven grains of chloride of zinc ; then coat two glass plates in 
the usual way, allow them to become quite dry, and immerse 
in a nitrate bath. Rear one of the plates on end to drain, but 
wash the other with water, and pour over it a little diluted 
albumen, afterwards drying a second time, and redipping in 
the bath. In the course of a few hours expose both to sun- 
light. It will be found that the chloride of silver upon collo- 
dion alone darkens to a violet-blue tint ; but that on albumen 
and collodion mixed changes to a deep olive-brown. ]N"ow 
apply a drop of nitric acid to the two plates ; the violet sub- 
chloride will be very little affected, but the brown image con- 
taining a suboxide will dissolve. Next treat the plates with 
ammonia; this will dissolve the violet and produce no effect 
upon the brown image. It is clear, therefore, from the action 



CHEMICAL CHANGES PEODUCED BY LIGHT. 101 

of these tests, that the presence of albumen makes an impor- 
tant difference in the nature and properties of the image. 

But in order to investigate the subject thoroughly, the dark- 
ened plates must be examined after, as well as before, treatment 
with a fixing agent, since a photographic picture cannot be 
said to be completed until it has passed through the bath of 
hyposulphite of soda. We therefore prepare a fresh series of 
films of chloride and nitrate of silver in the manner above 
described, viz., some on dry collodion and others upon albu- 
men, and having, by repeated treatment with fresh nitrate of 
silver, and a long exposure to the sun's rays, obtained the maxi- 
mum of darkening, we immerse them in a solution of hypo- 
sulphite of soda, wash carefully, and dry. Comparing the two 
sets of images after fixing, we find those on the collodion gray 
by reflected light, and pale slaty-bliie by transmitted light. 
The images on the albumen^ on the other hand, are of a dark 
olive-brown by reflection, and almost absolutely opaque when 
examined by transmission. The following tests are then ap- 
plied : a. Mercury: amalgamation with the collodion image, 
but none with the albumen, h. Cyanide of Potassiuni : no 
action on the gray and transparent image, but gradual solution 
of the brown opaque image, c. Sulphuretted Hydrogen Water : 
darkens the image on collodion, but gradually bleaches that on 
albumen, d. Permanganate of Potash, if in dilute solution, 
scarcely acts on the slaty-blue image, but oxidizes the brown 
image. 

Putting these facts together, we conclude that photographs 
formed on pure chloride and nitrate of silver consist of a sub- 
chloride of silver before fixing with hyposulphite of soda, and 
of metallic silver after fixing. In practice, however, it is 
found impossible to produce a picture on pure chloride of 
silver, the image being altogether too faint, and lacking depth 
of color by refiected light. When albumen, gelatine, or anal- 
ogous substances are present, the image contains suboxide of 
silver combined with organic matter, and this combination is 
not entirely destroyed in the process of fixing, for although 
both subchloride and suboxide of silver are instantly decom- 
posed by fixing agents, yet the latter does not appear to be so 
when united with albumen. 

The action of tests upon the organic image will vary with 
the length of time the j)lates have been exposed to light. The 
more intense the blackening the less evident is the action of 
solvents, such as the cyanide of potassium, etc. Hence the 
half-tones of a photographic picture often dissolve in fixing, 



102 THEORY OF PHOTOGRAPHY. 

whilst the full shadows remain. Unstable salts of gold, on 
the other hand, when applied to an image on paper, deposit 
the metal more readily on those parts which have been com- 
pletely blackened, and less so upon the lighter shades. Again, 
if photographic papers be prepared with two diiferent kinds 
of organic matter, as, for example, with Iceland moss in one 
case and albumen in another, the images obtained will not 
exactly correspond in properties. And if, in a third experi- 
ment, the paper be rendered sensitive upon a solution of am- 
monio nitrate of silver in place of nitrate of silver, the reaction 
with tests will again be different. Therefore, although the 
photograph contains both organic matter and silver, no certain 
formula can be given for its composition. 




REDUCING AGENTS. 103 



CHAPTER lY. 

ON THE DEVELOPMENT OF AN INVISIBLE IMAGE BY MEANS OF A 
REDUCING AGENT. 

It has been shown in the previous chapter that the majority 
of the salts of silver, either alone or in contact with organic 
matter, are darkened in color on exposure to light, and by the 
loss of oxygen, chlorine, etc., become reduced to the condition 
of subsalts. 

Many of the same compounds are also susceptible of a change 
under the influence of light, which is even more remarkable. 
This change takes place after a comparatively short exposure, 
and does not affect the appearance of the sensitive layer. The 
impression, however, although invisible at first, is brought out 
by treating the plate with certain chemical agents, which are 
without effect on the original unchanged salt. 

Experiments Illustrating the Formation of an Inmsible 
linage. — Take a sheet of sensitive paper, prepared with iodide 
of silver by the method given in another part of this work, and 
having divided it into two parts, expose one of them to the 
luminous rays for a few seconds. No visible decomposition 
takes place, but on removing the pieces to a room dimly illumi- 
nated, and brushing with a solution of gallic acid, a manifest 
difference will be observed ; the one being unaffected, whilst 
the other darkens gradually until it becomes black. 

Experiment II. — A prepared sheet is shielded in certain 
parts by an opaque substance, and then after the requisite ex- 
posure, which is easily ascertained by a few trials, treated with 
the gallic acid as before ; in this case the protected part re- 
mains white, whilst the other darkens to a greater or less ex- 
tent. In the same way, copies of leaves, engravings, etc., may 
be made, very correct in the shading, and much resembling 
those produced by the prolonged action of light alone upon the 
chloride of silver. 

A great economy of time is effected by employing a sub- 
stance like gallic acid to develop or bring out to view an in- 
visible image, in preference to forming the picture by the 
direct action of light, unassisted by a developer. This is well 
shown in the results of some experiments conducted by M. 



.104 THEORY OF PHOTOGEAPHY. 

Claudet in the Daguerreotype process ; he found that with a 
sensitive layer of bromo-iodide of silver, an intensity of light 
three thousand times greater was required if the use of a de- 
veloper was omitted, and the exposure continued until the pic- 
ture became visible upon the plate. 

To increase the sensitiveness of photographic preparations 
is a point of great consequence ; and indeed, when the camera 
is used, from the low intensity of the luminous image formed 
in that instrument, no other plan than the one above described 
would he practicable. Hence the advancement, and indeed 
the very origin of the photographic art, may be dated from 
the first discovery of a process for bringing out to view an in- 
visible agent by means of a reducing agent. 

The present chapter is divided into two sections, first, the 
properties of the substances usually employed in reducing the 
salts of silver, with their mode of action ; and second, the for- 
mation and development of the invisible image. 

Section I. 
The Various Substances Employed as Reducing Agents. 

The most important of the developers are as follows : Gallic 
acid, pyrogallic acid, and the protosalts of iron. 

GALLIC AND PYEOGALLIC ACIDS. 

a. Gallic Acid. — Gallic acid is obtained from gall nuts, 
which are peculiar excrescences formed upon the branches 
and shoots of the Quercus infectoria by the puncture of a 
species of insect. The best kind is imported from Turkey, 
and sold in commerce as Aleppo Galls. Gall nuts do not con- 
tain gallic acids ready formed, but an analogous chemical 
principle termed Tannic Acid, well known for its astringent 
properties, and employment in the process of tanning raw 
hides. 

Gallic acid is produced by the decomposition and oxidation 
of tannic acid, when powdered galls are exposed for a long 
time in a moist state to the action of the air. By boiling the 
mass with water and filtering whilst hot, the acid is extracted, 
and being sparingly soluble in cold water, crystallizes on 
cooling. 

Gallic acid occurs in the form of long silky needles, soluble 
in 100 parts of cold and 3 of boiling water ; they -are also 
readily soluble in alcohol, but sparingly in ether. The aqueous 



THE SALTS OF IRON. 105 

solution becomes moldy on keeping, to obviate which the addi- 
tion of a lump of camphor or a drop or two of oil of cloves 
is recommended. 

Gallic acid is a feeble acid, scarcely reddening litmus ; it 
forms salts with metallic oxides, but those of silver and gold 
are reduced by it to the metallic state. 

b. PyfogcilliG Acid {or PyrogalloT). — The term pyro, pre- 
fixed to gallic acid implies that the new substance is obtained 
by action of heat upon that body. At a temperature of about 
410 deg. Fahr., gallic acid is decomposed, and gives off a white 
vapor, which condenses in lamellar crystals. This is pyrogallic 
acid. 

Pyrogallic acid is very soluble in cold water, and in alcohol 
and ether; the solution decomposes and becomes brown by 
exposure to the air. By adding sulphite of soda to an aqueous 
solution it remains colorless for a long time. Tannin has a 
similar effect, but in a less marked degree. It gives an indigo- 
blue color with protosnlphate of iron, which changes to dark 
green if any persulphate be present. ns^^ajj 

Although termed an acid, this substance is strictly neutral ; 
it does not redden litmus paper, and forms no salts. It has an 
affinity for oxygen, which is greatly augmented by the pre- 
sence of an alkali ; a mixture of this kind made by adding 
liquor potasses to pyrogallic acid, is often employed for absorb- 
ing the oxygen contained in atmospheric air. 

Commercial pyrogallic acid is often contaminated with em- 
pyreumatic oil, and also with a black insoluble substance known 
as metagallic acid, formed when the heat is raised above the 
proper temperature in the process of manufacture. 

THE SALTS OF IKON. 

There are two oxides of iron which form salts, viz.,f'the 
protoxide of iron, containing an atom of oxygen to one of 
metal ; and the peroxide, with an atom and a half of oxygen 
to one of metal. As half atoms, however, are not allowed in 
chemical language, it is usual to say that the peroxide of iron 
contains three atoms of oxygen to two of metallic iron. 

Expressed in symbols the composition is as follows : 

Protoxide of Iron (Ferrous Oxide), Fe,0.] 
Peroxide of Iron (Ferric Oxide), FcgOa. 

The proto- and pcr-salts of iron do not resemble each other 
in their physical and chemical properties. The former are 



106 THEORY OF PHOTOGKAPHT. 

usually of an apple-green color, and their aqueous solutions are 
almost colorless, if not highly concentrated. The latter, on 
the other hand, are dark, and give a yellow or even blood-red 
solution. 

The following experiment will serve to illustrate the proper- 
ties of both classes of salts : — Take a crystal of protosulphate 
of iron, and having reduced it to powder, pour a little nitric 
acid upon it in a test-tube. On the application of heat, abun- 
dance of fumes will be given off, and a red solution obtained. 
The nitric acid in this reaction imparts oxygen, and converts 
the protosulphate entirely into a persalt of iron. It is this 
feature, viz., the tendency to absorb oxygen, and to pass into 
the state of persalts, which makes the protosalts of iron useful 
in photography. 

There are two protosalts of iron commonly employed by 
photographers : the protosulphate and the protonitrate of iron, 
the former being for wet collodion negatives, and the latter 
for positives. 

a. Protosulphate of Iron. — This salt, often termed copperas 
or green vitriol, is an abundant substance, and is used for a 
variety of purposes in the arts. Commercial sulphate of iron, 
however, being prepared on a large scale, requires recrystalli- 
zation to render it suflticiently pure for photographic purposes. 

Pure sulphate of iron occurs in the form of large transpar- 
ent, prismatic crystals, of a delicate green color : by exposure 
to the air tliey gradually absorb oxygen and they become rusty 
on the surface. Solution of sulphate of iron, colorless at first, 
afterwards changes to a red tint, and deposits a brown powder; 
this powder is a basic persulphate of iron, that is, a persul- 
phate containing an excess of the oxide or base. By the ad- 
dition of sulphuric or acetic acid to the solution, the forma- 
tion of a deposit is prevented, the brown powder being soluble 
in acid liquids. 

The crystals of sulphate of iron include a large quantity of 
water of crystallization, a part of which they lose by exposure 
to dry air. By a higher temperature, the salt may be ren- 
dered perfectly anhydrous, in which state it forms a white 
powder. 

b. Protonitrate of Iron. — This salt is prepared by double 
decomposition between nitrate of baryta or lead and protosul- 
phate of iron. It is an unstable substance, and crystallizes 
with great difficulty ; its aqueous solution is pale-greeii at first, 
but it is very prone to decomposition, even more so than the 
corresponding sulphate of iron. 



THE SALTS OF IRON. 107 

c. Proto-oxalate of Iron {Ferrous Oxalate). — For dry plates 
prepared with silver bromide, ferrous oxalate development is 
much employed. Allusion has been made to its preparation 
in the vocabulary, and practical details as to this and to its use 
will be found in a subsequent page. The solution of potas- 
sium oxalate, in which the ferrous oxalate is dissolved, takes 
no part in the development of the image, which being formed 
of metallic silver leaves ferrous bromide and ferric oxalate as 
the other products of the decomposition. 

Presence of Free Acids, etc., in Development. — Acids ex- 
ercise a retarding effect upon the reduction of salts of silver 
by the developing agents, and especially acids, like nitric or 
sulphuric acid, which have strong affinities for bases. Solution 
of pyrogallic acid mixed with nitrate of silver produces a de- 
posit immediately, and salts of iron will also separate silver 
before long; but if a little acetic acid be previously added, 
the precipitation is more gradual, and when both solutions are 
strongly acidified with nitric acid, it is for a time suspended. 
On the other hand, alkaline liquids produce an opposite effect, 
and favor quick reduction. 

Comparative Strength of Reducing Agents. — Sulphate of 
iron acts with rapidity. Kitrate of iron and gallic acid are 
both feeble reducing agents. Pyrogallic acid is stronger, a 
smaller quantity sufficing to produce the effect. Oil of cloves, 
grape sugar, aldehyde, honey, etc., are capable of reducing 
salts of silver, but they act too slowly to be employed with ad- 
vantage in photography. 

Effect of Temperature. — Reduction of the salts of silver 
proceeds more rapidly in proportion as the temperature rises. 
In cold weather it will be found that the development is slower 
than usual, and that greater strength of the reducing agent 
and more free nitrate of silver is required to produce the effect. 
The action of sulphate of iron, however, is considered to be less 
affected by depression of temperature than that of pyrogallic 
acid. 

On the other hand, if the heat of the atmosphere be exces- 
sive, the tendency to rapid reduction will be greatly increased, 
the solutions decomposing each other almost immediately on 
mixing. In this case the remedy will be to use acetic acid 
freely, or in place of it citric acid, which, as a retarding agent, 
is at least twenty times stronger than acetic acid. 

Varieties of Color in Deposited Silver. — The precipitate of 
metallic silver obtained by the action of reducing agents upon 



108 THEORY OF PHOTOGKAPHY. 

the nitrate, varies mncli in color and in general appearance. 
If gallic or pyrogallic acid be employed, it is a black powder; 
wliilst the salts ol iron, and especially the same with free nitric 
acid added, produce a sparkling precipitate, resembling what 
is termed frosted silver. Grape sugar and many of the essen- 
tial oils, such as oil of cloves, etc., separate the metal from 
ammonio-nitrate of silver in the form of a brilliant mirror 
film, and are often employed in silvering glass. 

In remarking upon these peculiarities in the molecular con- 
dition of precipitated silver, it should be observed that the 
appearance of a metal whilst in mass is no indication of its 
color when in the state of fine powder. Platinum and iron, 
both bright metals, and susceptible of a high polish, are dull 
and intensely black when in a fine state of division ; gold is of 
a purple or yellowish brown ; mercury is a dirty gray. 

Section II. 

The Formation and Development of the Latent Image. 

In forming an extemporaneous theory on the production 
of the photographic image by development, it would there- 
fore be natural to suppose that the process consisted in com- 
mencing a reducing action upon the sensitive surface by means 
of the luminous image of the camera, and afterwards complet- 
ing or carrying on the same by the application of the devel- 
oping solution. We may remark, however, that this hypothesis 
is not the one most strongly supported by facts, and that it 
will certainly lead us astray if too far pressed. The idea of 
the luminous image in the camera originating a chemical 
change, and the reducing agent carrying on that change to its 
completion, might lead to the inference that the two agencies 
were exactly similar in their mode of operation, and that the 
one could be substituted for the other; that when, for in- 
stance, the expcsure in the camera had been very brief, the 
defect would be remedied by prolonging the development. 
Such a notion is quite erroneous. A definite time is occupied 
in the formation of the invisible image, which cannot be 
shortened or extended beyond its proper limits with impunity. 
So far from the action of the developing solution being facili- 
tated when the plate has been left in the camera for an unusu- 
ally long time, it is often retarded thereby; and, on the other 
hand, unless the exposure be sufficient, it will be in vain to 
expect to produce a picture by strengthening the reducing 



NATURE OF THE INVISIBLE IMAGE. 109 

agent or keeping^it upon the plate for a longer time. Thus 
we see that "the invisible image" has a real existence, and 
that its formation by the solar ray is a phenomenon quite dis- 
tinct from any after process of development. 

HYPOTHESIS ON THE NATURE OF THE INVISIBLE IMAGE. 

When a sensitive iibn containing a salt of silver is exposed 
in the camera, and receives a latent impression capable of sub- 
sequent development, an inquiry arises as to the exact condi- 
tion of the film-particles on which the light has acted. The 
most careful inspection, even with the aid of a microscope, 
fails to detect any difference between the exposed and non- 
exposed portions of the plate. Chemical solvents likewise act 
in the same manner, both before and after the insolation, thus 
showing that there is no actual separation of the elements of 
the film, analogous to the elimination of chlorine from chloride 
of silver by the prolonged action of the sun's rays. 

The formation of the invisible image is supposed to be a 
molecular change, unattended with any separation of elements 
such as occurs in the case of a visible image impressed by 
light. The following diagrams will assist mechanically in 
conveying an idea of what we mean by a molecular change. 




Fig. 1. Fig. 2. 

Let Fig. 1 represent the iodide of silver in its normal state, 
and Fig. 2 the same iodide after its impression in the camera. 
The percentage composition has not been changed, for sup- 
posing one of the circles to represent iodine and the other 
silver, we have in both figures a single atom of each. The 
arrangement of the constituent atoms, however, has been 
modified in the second figure, where they are so placed as to 
touch each other only at the edges. Let it be borne in mind 
that this representation is purely hypothetical. We observe 
certain differences in the properties of the iodide after its 
insolation, and suppose them to indicate corresponding differ- 
ences in molecular ai-i-angeinent ; but we cannot speak with 
certainty, since the ultimate atoms of bodies are too minute to 
come within the rano^e of our observation. 



110 THEORY OF PHOTOGRAPHY. 

We now proceed to examine further the tangible differences 
between the impressed and the non-impressed atom of iodide 
of silver. If the two be exposed for a short time over mer- 
cury slightly warmed, the metallic vapor will condense upon 
the former, but not upon the latter ; and in this way the latent 
picture is made visible in the Daguerreotype process. So, 
again, if nitrate of silver, previously mixed with a reducing 
agent, be applied to the same two atoms, it will be seen that 
the impressed or modified iodide hastens the decomposition of 
the mixture, whereas the iodide prepared in total darkness 
exercises no such effect. We have indeed learnt from the 
observations in the last section that reducing agents them- 
selves decompose nitrate of silver, and throw down the metal 
in a pure form. This reduction, however, is usually slow and 
gradual, and when a little free acid, such as nitric or acetic 
acid, is added, the mixture remains perfectly clear for a period 
of five minutes or longer. Prepare, therefore, such a mixture 
with the proportions correctly adjusted, and drop into it two 
particles, one of the ordinary and the other of the impressed 
iodide of silver; when it wmII be found tliat the first remains 
yellow, but that the second immediately becomes black, and 
is incrusted with metallic silver. 

There are other instances of the surface of bodies under- 
going a molecular change, not to be detected by the eye, but 
yet sufficient to affect the afiinities of compounds brought into 
contact with such surfaces. 

An instructive illustration of this may be conducted as fol- 
lows : Take a clean glass vessel, and having poured into it a 
strong aqueous solution of chloride of potassium, add a little 
tartaric acid, and stir briskly with a glass rod. In a few mo- 
ments the parts of the glass touched by the rod will be marked 
by a line of minute crystals. In this case there is a disposition 
in the liquid to crystallize ; but the rubbed state of the glass, 
where the rod has touched it, facilitates the change, and deter- 
mines the point at which the crystallization shall commence. 
The deposit is adherent, and is not removed by emptying the 
capsule of its contents and washing it out with alcohol ; warm 
water, however, dissolves the crystals of bitartrate, and the 
surface of the glass is then found intact ; the rubijed condi- 
tion, as we have termed it, being temporary and quite invisible. 

One additional illustration of molecular in contradistinction 
to chemical change will suffice. When an engi-aving which 
has been framed and suspended for a length of time is taken 
down to be cleaned, it will sometimes happen that the glass, how- 



NATURE OF THE INVISIBLE IMAGE. Ill 

ever carefully polished by rubbing;, will exhibit the outlines of 
the picture when breathed on. The breath settles in certain 
parts and not in others, in consequence of variations in the 
surface condition, Wlien the glass is put aside the invisible 
image gradually vanishes, and the molecules return to their 
original state. 

Granting, therefore, that a ray of white light in acting on 
iodide of silver, so modifies it that it acquires the property of 
accelerating chemical change in the mixture known to photo- 
graphers as the developing agent, we observe, in the next 
place, that it is not necessary to suppose that the invisible 
modification is permanent, or that the particles of the iodide 



Fig. 3. Fig. 4. 

once disturbed, cannot afterwards be restored to their normal 
state. On the contrary, there are facts which prove that the 
invisible image may completely disappear, leaving the plate in 
a condition to receive a second impression in the camera. 

Mechanical diagrams may here be again employed, and their 
use will be free from objection, if it be understood that they 
are meant only to assist in fixing facts upon the mind of the 
reader, and, like a system of artificial memory, may be laid 
aside when this important object has been accomplished. 

In Fig. 8 we see two ovals, slightly different in size, and 
turning on a common centre; in {a) their long diameters cor- 
respond, but in {b) they cross each other, whilst in {c) they are 
made to coincide once more by a further action of the same 
force. In Fig. 4 we have the more familiar illustration of 
waves ; first an elevation, then a corresponding depression. 
Now, in the photographic process we find that the invisible 
effect of the light rises gradually to a certain pitch of intensity, 
but beyond that it appears to descend as it were on the other 
side ; hence the particle of iodide of silver first acquires the 
property of blackening the developer, but loses it again if left 
too long in the camera. 

THE DEVELOPMENT OF THE INVISIBLE IMAGE. 

The amateur cotnmencing the study of photography will 
probably form an incorrect idea of the real nature of a de- 
veloping agent. Being in the habit of applying to. the surface 



112 THEORY OF PHOTOGRAPHY. 

of the wet collodion a solution of pyrogallic acid or of sulphate 
of iron, he terms these compounds " the developers," and for- 
gets that a floating layer of solution of nitrate of silver is alfo 
present, derived from the bath. To impress upon the mind 
the fact that the developer is not the simple reducing agent, 
but the same mixed with nitrate of silver, let the following 
experiment be made : Coat a plate with iodized collodion, 
pass it through the nitrate bath, and wash it carefully, back 
and front, with distilled water. Now expose for an instant to 
diffused daylight, and apply the pyrogallic acid ; the plate will 
remain perfectly clear and transparent. The invisible impres- 
sion, however, is truly present, and the insolated iodide is 
ready to play its part, but no blackening takes place, because 
the reducing agent has nothing to act upon. Let the pyro- 
gallic acid, therefore, be poured back into a measure contain- 
ing a few drops of solution of nitrate of silver and applied a 
second time to the plate, when the tilm will begin to darken, 
and continue to do so until it is quite opaque. 

The Second Stage of the Development. — This consists in 
strengthening the image first formed by an additional deposit 
of silver. Take a sensitive collodion plate, and having im- 
pressed an invisible image upon it by a proper exposure in the 
camera, remove it to the dark room, and pour over it a solu- 
tion of pyrogallic acid. When the picture has fully appeared, 
stop the action by washing the plate with water. An exam- 
ination of the image at this stage will show that it is perfect 
in the details, but pale and translucent. Now take the plate 
and treat it to pyrogallic acid to which fresh nitrate of silver 
has been added; immediately the picture will become much 
blacker, and will continue to darken, even to complete opacity, 
if the supply of nitrate be kept up. The same result may be 
obtained after the iodide of silver has been removed from the 
plate by hyposulphite of soda or cyanide of potassium ; and in 
such a case it is evident that the additional deposit upon the 
image must be produced from the nitrate of silver, and not 
from the iodide of silver. Observe, also, that this additional 
deposit forms only upon the image, exhibiting no affinity for 
the unaltered iodide upon the part of the plate corresponding 
to the shadows of the picture, but attaching itself in prefer- 
ence to those parts already blackened by the developer. 

The second stage of the development, in which a feeble 
image is strengthened and rendei'ed more opaque, is a process 
bearing a close resemblance to the growth of a crystal in a 
saturated liquid, by aggregation of fresh particles ; and after 



OBJECTIONS TO THE FOKEGOING VIEWS. 113 

the picture lias readied its full density, a series of elevations 
may often be seen upon the plate, corresponding to the lines 
of the image. 

OBJECTIONS TO THE FOEEGOING VIEWS CONSIDERED. 

The theory of the formation and development of the latent 
image given in the preceding pages was not originated by the 
author of this work, nor can he at present specify any one 
individual to whom it can be ascribed. Objections liave been 
urged against it, and other hypotheses adopted in preference, 
not only in this country, bnt also in France. It has been said, 
for instance, that the process of photographic printing upon 
chloride of silver involves a partial reduction of the chloride 
by the action of ligiit, and hence it is reasonable to suppose 
that the iodide of silver suffers reduction in the developing 
process. We must bear in mind, however, that there is a great 
difference in the intensity of the light to which the sensitive 
surface is subjected in the two cases mentioned, and not only 
so, but that the iodide of silver is more difficult to reduce than 
the chloride. In fact, as we have already shown, the solar ray 
has no effect in discoloring pure iodide of silver, although it 
quickly blackens pure chloride of silver. If tlie invisible im- 
age were simply a disturbance of ultimate particles, rendering 
their after separation by the reducing agent more easy, we 
should expect to find chloride of silver receiving the image of 
the camera with greater rapidity than iodide of silver. The 
contrary, however, is the case, and we are probably within the 
mark in stating the sensitiveness of iodide of silver to be ten 
times greater than that of bromide, and sixty times greater 
than that of chloride of silver. 

Another argument militating strongly against the view often 
expressed, that the latent image consists of iodide of silver 
reduced in inappreciable quantities, or of iodide of silver with 
an increased tendency to reduction, is the fact before men- 
tioned, that the rapidity of the development is not in propor- 
tion to the length of the exposure in the camera. On the 
over-exposed sky of a landscape the iodide will sometimes 
remain yellow and unchanged throughout the whole process, 
whilst the less exposed parts of the plate darken under the 
influence of the developer. 

The opponents of the views which we have given of the 
purely molecular character of the disturbance in the camera, 
base their objections chiefly on phenomena observed in the 



114 THEORY OF PHOTOGRAPHY. 

practice of the dry collodion processes. It is not nniisual in 
such methods to obtain visible indications of the picture upon 
the surface of the lihii before the developer is ajiplied ; and 
even where no such indications present themselves, there yet 
often exists a remarkable difference between the exposed and 
the non-exposed parts to which as yet we have not directed 
the reader's attention. Mr. Young, of Manchester, was the 
first to notice the phenomenon, and to show that if an exposed 
plate be immersed in solntion of hyposulphite of soda until 
yellow iodide of silver is dissolved, a picture will still appear 
on applying the usual developing mixture. The effect cannot 
be obtained on a wet collodion plate, and only imperfectly on 
a plate preserved by honey, the best substance to exhibit it 
being albumen. Experiments have been made with a view of 
accounting for a phenomenon so remarkable as the develop- 
ment of an invisible image upon a plate previously cleared of 
its iodide of silver, and the explanation suggested is as fol- 
lows : Pure iodide of silver is molecularly modified by the 
camera image, but its properties as regards the action of hypo- 
sulphite of soda are unaffected thereby ; upon a plate so consti- 
tuted no image could be developed after fixing. The sensitive 
surface of the collodion film is never, however, an absolutely 
pure iodide of silver, and sometimes very far from being so. 
It contains traces of an organic compound of silver in the wet 
collodion process, and more appreciable quantities of the same 
in the dry processes. Now, with a compound film of this 
kind, it may be supposed that whilst the iodide of silver under- 
goes its peculiar change in the camera, the organic combination 
of silver likewise changes, but in a different manner. With- 
out altering in appearance, it gradually loses its solubility in 
fixing agents, so that when the hyposulphite of soda is applied 
its removal is not effected. We do not, however, perceive any 
picture upon the film, because the organic combinations of 
silver are transparent in thin layers, even when they contain 
iodide of silver. An albumino-iodide of silver, for instance, 
may be prepared in clear and colorless lumps like the finest 
jelly, and it is not difficult to conceive that such a substance 
may remain in an invisible condition upon Mr. Young's plates 
even after fixing. 

If the above hypothesis be correct, there exist two varieties 
of the invisible image : first, that upon simple iodide of silver, 
which enables it to condense mercury or tu determine chemi- 
cal change in the unstable developer; secondly, the image 
upon iodide of silver placed in contact with an organic com- 



SENSITIVENESS OF IODIDE OF SILVEK. 115 

pound of silver. In the latter case the movement of the 
particles of the iodide is supposed to be propagated to the 
other body, and changes are thus originated, visible or latent, 
according to the duration of the exposure, and the properties 
of the superadded substance. An organic salt of silver con- 
tains in itself the elements of its own reduction, and hence we 
are not surprised to find a catalytic action exerted upon it by 
the actinically excited iodide of silver. 

CAUSES WHICH AFFECT THE SENSITIVENESS OF IODIDE OF SILVEK. 

By the term sensitiveness we understand a facility of re- 
ceiving an impression from a very feeble ray of light, or of re- 
ceiving it quickly from a bright ray. This impression, how- 
ever, need not be followed by a vigorous or intense develop- 
ment, in order to constitute sensitiveness. On the contrary, 
it often happens that the most sensitive film yields a feeble 
picture, and when the iodide is so prepared as to produce an 
opaque picture, then it is less sensitive and requires to be ex- 
posed in the camera for a longer time. 

It has already been stated that although iodide of silver is 
less affected by direct sunlight than bromide or chloride of 
silver, yet that it is more sensitive to the reception of the in- 
visible image than either of those compounds. Placing iodide 
of silver, therefore, at the head of the list, we remark that, in 
the state in which it is used upon collodion, it possesses the 
highest degree of sensitiveness when there is an excess of 
nitrate of silver ; and if the experiment be made of washing 
the plate carefully with distilled water so as to remove the free 
nitrate, a longer exposure in the camera will be required. 
The sensitiveness does not, however, increase uniformly with 
the amount of free nitrate, and it has been found in the collo- 
dion process that no advantage can be gained by using a solu- 
tion of nitrate of silver stronger than that usually recom- 
mended for the bath. 

Strong acids, like nitric acid, greatly diminish the sensitive- 
ness of iodide of pilver. Even a weak vegetable acid, such as 
the acetic, has a similar though less decided effect, and if the 
collodion film be rendered very acid with acetic acid before 
its exposure in the camera, it will be found impossible to take 
a picture rapidly, even after strengthening the developer to 
the utmost limit. 

It has long been remarked that the use of bodies like al- 
bumen, gelatine, caseine, etc., which combine with oxides of 



116 THEORY OF PHOTOGEAPHY. 

silver, retards the action of light upon iodide of silver ; and one 
principal reason why the collodion iilm is so sensitive is be- 
lieved to be that pyroxyline, the basis of collodion, is a sub- 
stance peculiarly indifferent to the salts of silver, and exhibits 
very little tendency to combine with them. The photographer 
often employs albumen, but he does so with a view of increas- 
ing the opacity of the image, and not for the purpose of add- 
ing to the sensitiveness. Whilst discussing Mr. Young's ex- 
periments, we have distinguished between simple iodide of 
silver and iodide with addition of organic compounds. The 
latter combination gives the more vigorous picture, but the 
former is superior in sensitiveness. 

The great sensitiveness of iodide of silver in collodion may 
also be due in a measure to mechanical causes. The loose state 
of coagulation in a collodion film and the exquisitely fine state 
of division in which the particles are deposited must be favor- 
able to molecular and chemical change. It will be seen as we 
proceed that success in this process depends very much upon 
correctly balancing the different solutions, in order that the 
iodide of silver may be thrown down in a state favorable to 
penetration by the developer. 

Before passing to the next division of our subject, it may 
further be remarked, that in an}' photographic process in which 
an invisible image is produced, the time occupied in forming 
that image will vary more or less with the nature of the agent 
by which it is to be developed. Bodies like the protosulphate 
of iron, possessing a strong affinity for oxj'gen, will throw 
down metallic silver upon a surface of iodide of silver which 
has not undergone a sufiiciently decided modification in the 
camera to be affected by a weaker developer like gallic acid. 
It is probable that the future of instantaneous pliotography 
lies in the discovery of a developer even more unstable than a 
mixture of sulphate of iron and nitrate of silver; for the greater 
the instability, the less the need of a previously disturbing 
force to originate motion in the particles. 

The controlling influence of the developer upon the sensi- 
tiveness of iodide of silver, must be borne in mind instituting 
comparisons between the Daguerreotype and the collodion pro- 
cess. The employment of bromine in conjunction with 
iodine increases the sensitiveness of the Daguerreotype, but we 
have no reason to suppose that such would be the case if the 
image were developed in a different manner. The vapor of 
mercury condenses more readily upon the insolated bromo- 
iodide than upon the simple iodide of silver, but another vapor 



EESEAECHES ON INVISIBLE IMAGES. 117 

might condense less readily, and experiment is the only safe 
guide. 

Whilst upon this subject we also remark that the iodide of 
silver formed on a Daguerreotype plate ought not to be com- 
pared photographically with iodide of silver thrown down 
from aqueous sohitions. In the case of the metal plate there 
is a substratum of metallic silver, and it is not certain that the 
composition of this iodide corresponds to that precipitated in 
the moist way. The action of light upon it certainly diifers, 
for whereas the humid iodide remains unaffected in the sun's 
rays, the iodide of the Daguerrreot^■pist gradually becomes in- 
soluble in solution of hyposulphite of soda. 

KESEAECHES OF M. MOSEE AND OTHEES ON INVISIBLE IMAGES. 

The papers of M. Ludwig Moser " On the Formation and 
Development of Invisible Images," published in 1842, explain 
60 clearly many remai'kable phenoineiia of occasional occur- 
rence in the photographic processes that no apology need be 
offered for referring to them. 

His lirst proposition may be stated thus, viz., "If a polished 
surface has been touched in particular parts by anybody, it ac- 
quires the property of precipitating certain vapors on these 
spots differently to what it does on the other untouched parts." 
To illustrate this, take a thin plate of metal, having characters 
excised : warm it gently, and lay it upon the surface of a clean 
mirror glass for a few minutes: then remove, allow to cool, 
and breath upon the glass, when the outlines of the device will 
be distinctly seen. A plate of polished silver may be sub- 
stituted for the glass, and in place of developing the image by 
the breath, it may be brought out by mercurial vapor. 

The second proposition of M. Moser was as follows : " Light 
acts on bodies, and its influence may be tested by vapors that 
adhere to the substance." A plate of mirror glass is exposed 
in the camera to a bright and intense light ; it is then removed 
and breathed upon, when an image before invisible will be 
developed, the breath settling most strongly upon the parts 
where the light has acled. A plate of polished silver may be 
used as before instead of glass, the vapor of mercury or of 
water being employed to develop the image. A silver plate 
exposed to the vapor of iodine until iodide of silver has formed 
upon its surface, is still more sensitive to the influence of light, 
and receives a very perfect impression under the subsequent 
action of the mercury. 



118 THEORY OF PHOTOGRAPHY. 

The above, and other experiments of M. Moser have a prac- 
tical significance, for it has since been fonnd that the same con- 
dition of surface which causes a vapor to settle in a peculiar 
manner may also aJffect the behavior of the iodide of silver 
when treated with a mixture of nitrate of silver and a reducing 
agent. Thus, if a clean glass plate be touched in certain spots 
bj the warm finger, the impression soon disappears, but is again 
seen on breathing upon the glass ; and if this same plate be 
coated with a very delicate layer of iodized collodion and 
passed through the nitrate bath, the solution of pyrogallic 
acid will often produce a well-defined outline of the figure 
even before the plate has been exposed in the camera. This 
experiment is an instructive one, and shows the necessity of 
cleaning the plates used in photography with care. If there 
be any irregularity in the manner in which the breath settles 
upon the glass when it is breathed on, a condition of surface 
exists at that point which will probably so modify the layer of 
iodide of silver that the action of the developing finid will be 
in some way interfered with. 

Glass plates with collodion pictures on them should be 
cleaned very carefully before being again used, or the old im- 
pression will re-appear during development. Plates packed 
in sheets of newspaper often show the letters in the same way 
when the pyrogallic acid is applied : traces of organic matter 
in all probability are present on the surface of the glass, and 
it is only by long soaking in chemical solutions that these in- 
visible images can be destroyed. It may in every case be as- 
sumed that the existence of the invisible image can be detect- 
ed by breathing, and that a glass which does not affect the 
breath is photographically clean. 

M. Moser in the same series of papers calls attention to the 
peculiar phenomena resulting from over-action of light. " If 
light," he says, " acts on iodide of silver, it imparts to it the 
power of condensing mercurial vapors, but if it acts beyond a 
certain time, it then diminishes this power, and at length takes 
it away altogether." This observation, like the last, is a prac- 
tical one, for in all photographic processes we have to contend 
against the phenomenon known as solarization. Over-expos- 
ure in the camera will invariably weaken the action of the de- 
veloper more or less, but nevertheless the tendency to solarize 
may be overcome in a measure by associating other substances 
with the iodide of silver, so as to impart greater stability to the 
invisible image. Bromide of silver appears to exercise such 
an effect, as also does an acid condition of the film. The pres- 



EESEAKCHES ON INVISIBLE IMAGES. 119 

ence of organic salts of silver in the film likewise influences the 
amount of solarization. 

Observations by Norris and Others. — When a sensitive 
collodion plate is washed in water before its exposure in the 
camera, a mixture of pyrogallic acid and nitrate of silver is re- 
quired to develop it. The mixture is usually made before the 
developer is applied to the film, and this is of more importance 
than it would appear. If the pyrogallic acid solution be used 
alone without any nitrate of silver, it will frequently obliterate 
the latent impression, so that a second picture may be taken 
upon the same plate. Such, however, is not invariably the 
case; if a mere trace of nitrate of silver remain in the pores 
of the collodion, there will be development instead of oblitera- 
tion. A washed collodion film may have its free nitrate of 
silver restored to it by dipping in the bath after exposure, but 
it will, as a rule, develop with unusual energy after such treat- 
ment, and the picture will be taken by a shorter exposure than 
if the plate were simply flooded with a mixture of pyrogallic 
acid and nitrate of silver. 

Dilute acids gradually obliterate the latent image on washed 
collodio-iodide of silver. When an exposed plate is left for a 
time over the vapor of acetic acid, the impression gradually 
disappears, and the film returns to a sensitive condition. On 
repeating this experiment, the author finds that the second 
latent image requires a longer exposure in the camera, and de- 
velops in a somewhat different manner from the first. 

Dry iodide of silver on collodion gradually loses its power 
of receiving the latent image, when placed in contact with cer- 
tain other bodies. Thus the extreme edges of dry plates, 
where they touch the grooves of the carrying-box, often re- 
main transparent on developing, and exhibit no indications of 
an impression. Traces of organic matter have a similar effect 
on the iodide in the wet process, as may be proved by allow- 
ing a drop of saliva to fall on a glass plate, and then carefully 
removing it with a silk handkerchief. On collodionizing the 
glass and passing it through the nitrate bath, there will be a 
transparent mark of defective development on that particular 
part, showing that the iodide had not undergone the usual 
change in the camera. 

Grove's JElect7'ical Images. — Mr. Grove has succeeded in 
producing latent images by electricity. In the experiments 
described, a plate of glass was electrized in certain parts, and 
then breathed on, or exposed to the fumes of the hydrofluoiic 
acid ; in either case the vapor settled exclusively upon the 



120 THEORY OF PHOTOGEAPHY. 

w<9w-electrical part of the glass. When the glass was first 
electrized, and afterwards coated with iodide of silver and ex- 
posed to light, pyrogallic acid produced no reduction. 

Experiments of Bush and Others. — Mr. Busk has stated 
that ordinary writing paper sensitized with nitrate of silver 
undergoes an invisible change when placed in contact with 
certain surfaces ; and that in consequence of this change the 
paper loses its property of being darkened by exposure to 
light. To exhibit the phenomenon in perfection, he recom- 
mends that the nitrate bath should be rather strongly acidified 
with tartaric or acetic acid. A few hours' contact with an en- 
graving will then produce the effect, and those portions of the 
sensitive paper which have been touclied by the blacks of the 
engraving will remain wiiite in the solar ray, whilst the other 
untouched parts will darken in the nsual manner. 

Mr. Malone observes, with reference to the above, that much 
depends upon the composition of the coloring-ink of the en- 
graving, and that one kind of ink will suspend the reducing 
process in a sensitive paper, whilst another will fail in doing 
so. His mode of procedure is as follows : The paper is 
rendered sensitive upon a neutral nitrate bath, and is then 
placed between the leaves of a book. Spontaneous darkening 
gradually ensues, but the parts opposite to the black letters re- 
main white, in consequence of some unexplained infiuence 
exerted by the ink. 

Invisible Images Produced hy the Agency of Ozone. — M. 
Thenard has shown that ordinary paper exposed to a current 
of ozonized oxygen, experiences a change not visible to the 
eye, but becoming so when the paper is laid in contact with a 
sensitive sheet containing chloride and nitrate of silver ; 
blackening of the nitrate takes place opposite to the ozonized 
surface. When the ozonized paper is enclosed in a damp tube, 
a peculiar odor is gradually developed, due in all probability 
to an oxidation of the organic matter into a body resembling 
aldehyde or formic acid in its property of reducing nitrate of 
silver to the metallic state. These experiments of M. Thenard 
were undertaken in consequence of the publication of a series 
of observations by M. Niepce de St. Yictor, attributing similar 
effects to the agency of latent light. The ingenuity of M. 
Niepce is praiseworthy, but the opinion entertained at the 
present time appears to be that the phenomena he has des- 
cribed are susceptible of a simpler explanation than that in- 
volved in the supposition of stured-up light. 



NATURE OF DEVELOPED IMAGES. 121 



CHEMICAL NATURE OF DEVELOPED IMAGES, . 

Photographic pictures obtained by developing an in- 
visible image with a reducing agent, contain usually a far 
larger quantity of metallic silver than those produced 
by the iong-continued action of light alone. Hence they 
are less easily injured by destructive agents, such as 
cyanide of potassium, oxidizers, etc. The plan which has 
been adopted to ascertain the actual quantity of silver 
present in an image, is first to convert the deposit into 
chloride of silver, and afterwards into sulphide of silver, by 
immersion in a solution of a soluble sul23hide. The more 
opaque the image appears after this treatment, the greater the 
quantity of real silver. 

Developed images vary, however, materially in their nature 
and properties, according to the circumstances under which 
they were produced. Photographic prints, for instance, taken 
on chloride of silver, by development after a moderate amount 
of 'exposure, resemble very closely images obtained by the 
direct action of light alone, whilst images like collodion 
positives are essentially different, and do not react in the same 
manner with tests. The former, indeed, may be compared to 
images formed by direct sunlight upon chloride of silver com- 
bined with organic matter; but the latter to images upon pure 
chloride of silver, which, as before shown, consist, after fixing, 
of metallic silver only. The following conditions may be men- 
tioned as affecting the character of the developed image : 

a. The Surface used to Sustain the Sensitive Layer. — Albu- 
men and gelatine are favorable to the production of a brown 
and opaque image, which will bear magnifying without show- 
ing separate particles. Cyanide of potassium and oxidizing 
agents dissolve this image, and it will not amalgamate with 
mercury. It is probably not metallic, but partly organic iu 
its nature. Collodion, on the contrary, often gives a gray 
and slaty image, the ultimate particles of which are seen on 
magnifying. It amalgamates with hot mercury, but is 
little affected by fixing agents and oxidizers. This image is 
nearly or quite metallic. 

Certain organic matters may be dissolved in the collodion, 
and the result will be to produce an image intermediate in 
properties between the two last described. Glycyrrhizine and 
grape sugar may be mentioned in illustration. 

b. The Nature of the Sensitive Salt. — Bromide of silver 
added to the iodide in an albumen or a gelatine process, pro- 



122 THEOKT OF PHOTOGRAPHY. 

duces an image with increased opacity, and one which, from 
the action of tests, we conchide to be more organic in its 
nature. Bromide of silver, in collodion, however, diminishes 
the intensity, and renders tlie image gray and metallic. In 
collodion containing certain kinds of organic matter purposely 
added, bromide behaves in the same manner as in albumen, 
increasing the opacity of the developed image. 

In the case of chloride of silver employed to receive a latent 
image, and supported by a surface of gelatine or albumen, the 
impression is usually slightly visible before development ; and 
when such is the case, the resulting picture will be organic in 
its nature, and behave with tests more in the manner of an 
ordinary photograj)hic paper darkened by light. 

c. The Developing Agent Am,ployed.— li\\\^ subject has 
already been noticed. Organic developers, gallic and pyro- 
gallic acids, tend to produce images of the opaque kind ; but 
the inorganic protosalts of iron, gray and metallic images. 
Acetate of iron resembles pyrogallic acid in its action more 
than it resembles the sulphate of iron. 

The addition of a strong acid like nitric acid to the developer 
produces a deposit, the particles of which are large and crystal- 
line, with small opacity by transmitted light; but an alkaline 
condition of the solutions is favorable to a brown deposit, with 
the particles in a state of fine division. 

d. The Stage of the Developunent. — The red image first 
formed on the application of the developer to a gelatinized or 
albumenized surface of iodide of silver is less metallic, and 
more easily injured by destructive tests, than the black image, 
which is the result of prolonging the action. Developed 
photographic prints, when of a bright red color after lixmg, 
correspond in properties to those obtained by the direct action 
of light on paper prepared with chloride of silver, and less 
closely to collodion, or even to fully developed Talbotype 
negatives. 

e. The Intensity of the Light Ity which the Invisible Image 
was Formed. — A very strong light acting for a short time is 
followed by the development of the red image, the particles of 
which are finely divided, and easily acted on by solvents and 
oxidizing agents. A dull light, on the other hand, acting for a 
longer time, is succeeded by an image of the metallic variety, 
which amalgamates with mercury. The two varieties of de- 
posit may often be seen on the same picture, the former in the 
high lights, as the sky ; the latter in the deep shadows where 
the chemical rays are feeble. 



ILLTJSTEATIONS OF ABNORMAL DEVELOPMENT. 123 



ILLUSTRATIONS OF ABNORMAL DEVELOPMENT. 

The characteristics of the proper development of a latent 
image are, that the action of the reducing agent should cause 
a blackening in the parts touched by light, but produce no 
effect upon those which have remained in shadow. In 
operating both on collodion and paper, liowever, we find a 
liability to failure in this respect, the film beginning, after the 
application of the developer, to change in color to a greater or 
less extent over the whole surface. 

Let us see how the above defect, known to the photographer 
as "fogging," may be originated. Supposing the invisible 
image to be duly formed, it is yet necessary for its correct 
development that a proper balance should be maintained be- 
tween the constituents of the developer. The reducing agent 
must not be too powerful, nor the quantity of nitrate of silver 
too large, otherwise, although tiie deposited silver will fall 
principally upon the image, a portion of it will be thrown 
down upon the shadows. It will also be found that the quan- 
tity of nitrate of silver being correctly balanced, foggifig will 
certainly ensue if a little of this nitrate be changed into oxide 
of silver by the addition of an alkali, since the tendency to 
reduction is then so strong that it cannot be controlled. Even 
when the nitrate is accurately neutral, care and avoidance of 
all disturbing causes will be required to prevent a deposition 
of silver upon the shadows of the image, especially when 
nitrate of silver or acetate of silver are present, both of which 
salts are more easily reduced than the nitrate of silver. 

The use of acid is the principal resource in obviating cloudi- 
ness of the image. Acids lessen the facility of reduction of 
the salts of silver by developing agents, and hence when they 
are present the metal is deposited tnore slowly, and only on 
the parts where the action of the light has so modified the 
particles of iodide as to favor decomposition ; whereas, if 
acids be absent, or present in insufficient quantity, the equilib- 
rium of the mixture of nitrate of silver and reducing agent 
which constitutes the developer is so unstable, that any rough 
point or sharp edge becomes a centre from which the chemical 
action, once started, radiates to all parts of the plate. 

Observe, however, that although we employ free acids in 
the developer to regulate the process and prevent the chemical 
actions from running riot, yet if we carry the proportion of 
the acid too far, or employ too strong an acid, we may orig- 
inate the very defect we wish to avoid. Excess of nitric acid 



124: THEOKY OF PHOTOGRAPHY, 

is a cause of fogging, if not as potent as excess of alkali, jet 
very decided. In this case the opposition to reduction is so 
strong, that for a long time after applying the developer to 
the plate no silver is deposited ; eventually spangles of metal 
are seen, which adhere like sparkling dust to the shadows. 

A more or less evident amount of fogging must be antici- 
pated when the exposure in the camera has been too short. 
The latent image being in such a case very weak and imper- 
fectly formed, does not act properly in attracting the metallic 
silver from the developer. When nitrate of silver and a 
reducing agent are present at the same time upon the film, 
metallic silver must very shortly fall upon some portion of 
the plate ; hence the best security against fogging is a vigorous 
latent image, because in that case the action is so rapid upon 
the exposed parts that the energy of the developer is soon 
spent. Those films of iodide of silver which are prepared with 
especial reference to the production of vigorous images always 
give clean pictures, whereas the more sensitive iodide of silver 
is very liable to fog, since the latent image, although quickly 
formed, is not very deep or decided. 

Spots upon the plate are instances of abnormal develop- 
ment, which may often be explained in a similar manner. In 
such a case the latent image upon the iodide of silver is not 
always sufficiently vigorous to exhaust with rapidity the mix- 
ture of reducing agent and nitrate of silver which constitutes 
the develoj^er. Each spot exhibits, it is true, a central nucleus 
of extraneous matter, but this does not invalidate the argu- 
ment. Such microscopic nuclei are always present upon the 
sensitive film to a greater or less extent, but if the invisible 
image be well formed, they are not so liable to increase in 
size during development. Full occupation must, so to speak, 
be provided for the developer, otherwise it will search out 
these minute particles and surround them with deposited 
silver. Filtering of solutions and avoidance of dust are useful 
remedies for spots, but so, likewise, is attention to the state of 
the sensitive film as regards vigorous and intense develop- 
ment. If, however, the energy of development be too great, 
the spots may recur ; just as fogging of the film is sometimes 
due to deficiency of acid, and sometimes to excess of acid. 



CHAPTER Y. 

FIXING AGENTS. 

A SENSITIVE layer of chloride or iodide of silver on which 
an image may be formed, either with or without the aid of a 
developing agent, must pass through further treatment in 
order to render it indestructible by diifused light. The image 
itself is sufficiently permanent, and cannot be said, in correct 
language, to need fixing ; but the unchanged silver salt which 
surrounds it, being still sensitive to light, tends to be decom- 
posed in its turn, and, by darkening, to obliterate the picture. 

In order that any body may be employed with success as a 
fixing agent, it is required not only that it should dissolve un- 
changed chloride or iodide of silver, but that it should pro- 
duce no injurious effect upon the image. A solvent action 
upon the image is most liable to happen when the agency of 
light alone, without a developer, has been employed ; in that 
case, the darkened surface, not being reduced perfectly to the 
metallic state, remains soluble to a certain extent in the fixing 
liquid. 

AlklMONIA AND OTHER FIXING AGENTS. 

Ammonia. — Ammonia dissolves chloride of silver readily, 
but not iodide of silver; hence its use is necessarily confined 
to the direct sun proofs upon paper. Even these, however, 
cannot advantageously be fixed in ammonia unless a deposit 
of gold has been previously produced upon the surface by a 
process of "toning," presently to be explained ; an unpleasant 
red tint is always caused by ammonia acting upon the darkened 
material of a sun picture as it comes from the printing-frame. 

The principal objections to the use of ammonia as a fixing 
agent are its pungent odor, and also the fact that it is scarcely 
strong enough in its affinities to dissolve the oxide of silver 
when associated with albumen and similar bodies. 

Chloride and Iodide as Fixing Agents. — The chlorides of 
potassium, ammonium, and sodium possess the property of 
dissolving a small portion of chloride of silver. In the act of 
solution a double salt is formed, a compound of chloride of 



126 THEOKY OF PHOTOGRAPHY. 

sodium with chloride of silver, which may be crystallized out 
by allowing the liquor to evaporate spontaneously. The ear- 
lier photographers employed a saturated solution of common 
salt for fixing paper prints ; but the fixing action of the alka- 
line chloride is very slow and imperfect. 

The iodide of potassium has been used as a fixing agent for 
iodide of silver. It dissolves it by forming a double salt in 
the manner before described. 

It is important to remark in the solution of the insoluble 
silver salts by alkaline chlorides, iodides, etc., that the amount 
dissolved is not in proportion to the quantity of the solvent, 
but to the degree of concentration of its aqueous solution. 
The reason is that the double salt formed is decomposed by a 
large quantity of water. Hence it is only a saturated solution 
of chloride of sodium which possesses any considerable power 
of fixing ; and with the iodide of potassium the same rule 
holds good — the stronger the solution the more iodide of 
silver will be taken up by a given weight. The addition of 
water produces milkiness and a deposit of the silver salt pre- 
viously dissolved. 

HYPOSULPHITE OE THIOSULPHATE OF SODA AS A FIXING AGENT. 

Thiosulphuric acid, 1128203, formerly called hyposulphurous 
acid, S2U2, is one of the oxides of sulphur. It is, as its name 
implies, of an acid nature, although the acid itself is scarcely 
known, for it cannot be isolated. 

The hyposulphite of soda emplo^^ed by photographers is se- 
lected as being more economical in preparation than other hy- 
posulphites adapted for fixing. It occurs in the form of large 
translucent groups of crystals, which include five atoms of 
water. These crystals are soluble in water almost to any ex- 
tent, the solution being attended with the production of cold ; 
they have a nauseous and bitter taste. 

Hyposulphite of soda is now made on an extensive scale, 
and sold at a low price. It is usually sufficiently pure in the 
state in which it comes from the manufacturers. 

In the solution of silver compounds by hyposulphite of 
soda a double decomposition always takes place ; thus : 

Hyposulphite of Soda + Chloride of Silver 
=Hyposulphite of Silver + Chloride of Sodium. 

The hyposulphite of silver combines with an excess of hy- 
posulphite of soda, and forms a soluble double salt, which may 



CYANIDE OF POTASSIUM AS A FIXING AGENT. 127 

be crystallized out by evaporating the solution. This com- 
pound pos?!essesan intensely sweet taste, and contains one atom 
of hyposulphite of silver, with two of hyposulphite of soda. 
In addition to this there is a second donble salt, differing from 
the first in being very sparingly soluble in water; it contains 
single atoms of each constituent. 

The quantity of chloride of silver which hyposulphite of 
soda will easily dissolve may be stated roughly as about one- 
third of its weight. If the proportion of chloride be increased 
to more than one-iialf of the hyposulphite, there will usually 
be an abundant deposition of minute sparkling crystals, since 
the second or sparingly soluble double salt of hyposulphite of 
silver and soda will then be formed. 

When hyposulphite of soda is employed as a fixing agent it 
is a Sf^fe precaution to convert any nitrate of silver which may 
be present into chloride of silver, otherwise there will be a 
danger of the solution becoming discolored from a decompo- 
sition into sulphite of silver. This cliange will be explained 
more particularly in the section on the fixing of paper proofs. 

Iodide of silver is dissolved by hyposulpliate of soda more 
slowly than chloride of silver, and the amount eventually taken 
up is far less — one part by weight of iodide of silver requiring 
about twenty-four parts by weight of hyposulphite of soda in 
a cold solution ; but if the solution be heated a much larger 
quantity of iodide of silver is dissolved. 

CYANIDE OF POTASSIUM AS A FIXING AGENT. 

The cyanide of potassium is the salt most frequently em- 
ployed in fixing, but cyanide of sodium will answer the pur- 
pose equally well. Cyanide of potassium, the commercial 
manufacture of which is described in the vocabulary, occurs 
in the form of fused lumps of considerable size. In this state 
it is usually contaminated with a large percentage of carbonate 
of potash, amounting in some cases to more than half its 
weight. By boiling in proof spirit the cyanide may be ex- 
tracted and crystallized, but this operation is scarcely required 
as far as its use in photography is concerned. 

Cyanide of potassium absorbs moisture on exposure to the 
air. It is very soluble in water, but the solution decomposes 
on keeping; changing in color and evolving the odor of prus- 
sic acid, which is a cyanide of hydrogen. Cyanide of potas- 
sium is highly poisonous, and must be used with caution. 

Solution of cyanide of potassium is a most energetic agent 



128 THEORY OF PHOTOOEAPHr. 

in dissolving the insoluble silver salts ; far more so than the 
hyposulphite of soda. The salts are in all cases converted into 
cyanide, and exist in the solution in the form of a soluble dou- 
ble salt, which, unlike the double sulphocyanide, is not decom- 
posed by dilution with water. Cyanide of potassium is not 
adapted for fixing positive proofs upon chloride of silver ; and 
even when a developer has been used, unless the solution of 
the cyanide is tolerably dilute, it is apt to attack the image, 
converting it superficially into cyanide of silver, and then dis- 
solving it in the form of a double cyanide of potassium and 
silver. 

The solvent powers of cyanide of potassium on metallic 
silver are much increased by the addition of a little iodine to 
its aqueous solution ; a colorless liquid is formed, which has 
been termed " lodo-cyanide of Potassium." 

SULPHOCTANIDES AS FIXING AGENTS. 

Sulphocyanides of potassium and ammonium have been pro- 
posed as fixing agents; they resemble the alkaline chlorides 
used for this purpose, inasmuch as their solvent power de- 
pends on the degree of concentration of the solution, but they 
greatly exceed the latter salts in their power of dissolving the 
insoluble salts of silver, when strong solutions are used. 

Though inferior to hyposulphites in this varying degree of 
solvent power, they have the advantage of imparting no sul- 
phur to the print, and they preserve the tones clear and free 
from mealiness. They are not in favor with practical photog- 
raphers. 

OBSEEVATIONS ON THE RELATIVE AFFINITIES OF CHLORINE, 
IODINE, AND HYPOSDLPHUEOUS ACID FOR SILVER. 

llyposulphurous (tliiosulphuric) acid is usually viewed as a 
body possessing extraordinary affinities for silver, inasmuch 
as a solution of hyposulphite of soda decomposes both iodide 
and chloride of silver. It may be shown, however, on the 
other hand, that a solution of a chloride or an iodide will 
decompose hyposulphite of silver under some circumstances ; 
and hence the relative affinities are less evident than may 
appear. The use of these fixing agents, indeed, depends 
upon their being present in large excess over the salt of silver 
to be dissolved, in order that a true double salt may be 
formed. When two different fixing agents are employed in a 



RELATIVE AFFINITIES. 129 

state of mixture, that particular one may prevail which is 
present in largest quantity, even although its aflSnities for 
silver are weaker than those of the other. 

Thus, although chloride of silver dissolves easily in solution 
of hyposulphite of soda, we find that hyposulphite of silver is 
decomposed and converted into chloride of silver when treated 
with a saturated solution of chloride of sodium. And the 
same remark applies to iodide of silver ; it is decomposed by 
a large excess of hyposulphite of soda, but not by a small 
excess. The addition of iodide of potassium to a fixing bath 
of hyposulphite nearly saturated with silver salt, will precipi- 
tate a portion of the silver in the form of the yellow iodide of 
silver. 




CHAPTER VI. 

THE MANUFACTURE OF PHOTOGRAPHIC COLLODION, 

In order to conduct the manufacture of collodion with suc- 
cess, a number of minor points must be considered, which 
could not conveniently be included in the general description 
already given. These relate, not only to the pyroxyline, but 
also to the solvents and the iodizing compounds; they will 
now be explained with as much fullness as possible. 

The most important point is the composition of the nitro- 
sulphuric acid, in which the pyroxyline is prej3ared ; for it 
will be found that the slightest variation in the quantity of 
water, in tiie relative proportions of the two constituent acids, 
or in the temperature at the time of putting in the cotton, will 
affect the result. It will be advisable, therefore, to examine 
these matters separately, and in addition to sj)eak of the condi- 
tion of the fibre itself, which is important both chemically and 
mechanically. 

a. Effect of Varying the Proportion of Water in the Nitro- 
SuljphuriG Acid. — When many separate quantities of pyroxy- 
line are made in the same nitro-sulphnric acid, the mixture 
becomes gradually weaker by the abstraction of the elements 
of water from the fibre, and each successive portion of pyroxy- 
line is less in the state of explosive gun cotton, and more in 
that of xyloidine, than the one which preceded it. 

There is considerable difference in the physical properties 
of collodions made from strong and weak samples of pyroxy- 
line prepared as above described. In the first instance, when 
the product is only one step removed from gun cotton, the 
collodion pours slowly upon the glass, and tends to thicken at 
the edges. The film is not very adherent, and will sometimes 
split away on drying. In attempting to coat a large plate, a 
wavy appearance, often known as woolliness of the film, is 
seen at the lower corner. With pyroxyline made in weak 
acids, on the other hand, the collodion is more limpid and free 
from structure, the film of iodine, when formed in the bath, 
presenting an even appearance throughout. 



MANUFACTUEE OF COLLODION. 131 

When the amount of water in the nitro-sulphnric acid is 
carried still further, one of two things happens; either the 
cotton instantly dissolves in the acid, or it is more or less dis- 
integrated without actually dissolvitig. The product in the 
latter case is not entirely soluble in ether and alcohol, but 
leaves behind a residue consisting in part of unaltered cotton 
fibre. 

Here it may be proper to explain the reason why a weak 
nitro-snlphuric acid sometimes dissolves the cotton, and some- 
times merely disintegrates it. Supposing the temperature and 
the cotton used to be the same, yet the solvent action of the 
acid will vary with the relative proportions of sulphuric and 
nitric acid present, even although the amount of water be cor- 
rectly adjusted to that of the sulphuric acid. Thus a warm 
diluted nitric acid used alone acts upon the cotton rapidly and 
dissolves it ; but if a very small portion of diluted sulphuric 
acid be added, the tendency to dissolve the cotton is lessened, 
or if the cotton be already dissolved, the diluted sulphuric acid 
throws it down again. If, however, the proportion of the 
diluted sulphuric acid be very considerably increased, until it 
reaches to as much as three times the bulk of the diluted 
nitric acid, then the tendency to dissolve the cotton becomes 
greater than in the case of a mixture containing equal volumes 
of the two diluted acids. 

b. A Peculiar Action of the Oil of Vitriol in the Process. 
— To demonstrate this, take a sheet of ordinary bibulous paper, 
and having cut it into square pieces, float them upon sulphuric 
acid, diluted with half its bulk of water and cooled ; allow 
five seconds for the first, ten for the second, twenty for the 
third, and so on, until the last piece is gelatinized and dis- 
solved ; then remove the sulphuric acid carefully by washing, 
and convert them into pyroxyline. A marked difference will 
be perceptible between the samples of collodion so obtained. 
The first noticeable effect of the previous parchmentizing will 
be an increased fluidity and freedom from structural lines. 
The collodion, when poured upon a glass, sets very rapidly, 
and with such firmness that the finger may be rubbed back, 
ward and forward without disturbing it. The film on lifting 
from the bath soon becomes partially surface-dry, and repels 
developers or fixing agents ; when washed with water and 
dried, it forms a dense and highly-varnished surface nearly im- 
prenetrable by liquids. The moist film, after development 
with pyrogallic acid and fixing, appears unusually tough, and 
will bear pumping on without injury. It is also very con- 



132 THEORY OF PH(JTOGEAPHY. 

tractile, and tends to draw itself away from the edges of the 
glass. When pushed aside it can be pulled back again like 
the finger of a glove. The fixing agent never removes any 
portion of the image from this collodion because the iodide of 
silver is in the film, and not only upon its surface. 

The above-mentioned properties, imparted by the prelimin- 
ary action of oil of vitriol, are not seen to an equal extent in all 
the pieces of paper used in the experiments, but are more de- 
cidedly evident in proportion to the time during which the 
acid was allowed to act. If, however, the action of the acid 
be carried to that point at which the paper begins to soften 
and become semi-gelatinous, then the resulting collodion will 
be entirely difierent,"the film being rotten and powdery. This 
we shall presently show to be due to a disintegration by the 
nitric acid, contained in the nitro-sulphuric acid, which dis- 
integration is exhibited more strongly when the fiber is pre- 
viously changed, nearly into dextrine, by the prolonged action 
of sulphuric acid. 

In addition to these effects produced by parchmentizing the 
fiber in difl^erent degrees before converting it into pyroxyline, 
there are others which deserve notice. The solubility of the 
product in ether and alcohol is much increased, so that the 
exact amount of water in the nitro-sulphuric acid becomes a 
point of less importance as affecting solubility, and an acid 
strong enough to make ordinary paper explosive and in- 
soluble ; answers perfectly in the case of paper previously 
parchmentized. 

To secure the parchmentizing action in the ordinary process 
of making pyroxyline, the amount of diluted sulphuric acid 
present in the nitro-sulphuric acid should be considerably 
greater than that of the nitric acid. The cotton then shrinks 
into a small compass, and the resulting collodion will be tough 
and strong. The action of the oil of vitriol in the process evi- 
dently precedes that of the nitric acid, since we find that cot- 
ton fibre which has once been converted into pyroxyline is no 
longer affected by diluted sulphuric acid, even when im- 
mersed for several hours. 

A Peculiar Action of the Nitric Acid in the Process of 
making Pyroxyline. — The proper action of nitric acid in this 
process is, as before shown, to communicate peroxide of nitro- 
gen to the fibre, and so to convert the cellulose into p3rroxy- 
line ; the stronger the nitric acid, the greater the amount of 
peroxide imparted. We notice, however, another action of 
nitric acid upon pyroxyline, by which the properties of the 



MANUFACTURE OF COLLODION. 133 

latter are much altered. This second, or modifying action, if 
we may so term it, is exerted not so much by a concentrated, 
as by a diluted nitric acid, and more by a hot nitric acid than 
by the same acid employed cold. To exhibit this secondary 
action of nitric acid, take ordinary pyroxyline made by the 
formula given in another part of this work, and dip it for an 
instant in nitric acid of sp. gr. 1.45, mixed with a 
third of its bulk of sulphuric acid (to prevent it from dis- 
solving the cotton) and heated to about 140 or 150 deg. 
Fahr.* The following are the main characteristics of col- 
lodion prepared from pyroxyline so treated : It does not set 
rapidly upon the glass like the parchmentized collodion last 
described, but remains liquid for a minute, or even longer, 
after which it rubs under the finger in a soapy manner, instead 
of bearing friction without injury. After passing through 
the bath the film is creamy, retains its surface moisture for an 
unusually long time, and on washing with water and drying 
presents a porous surface, quite lustreless, and without any 
varnished appearance. This film, when wet, will not bear 
pumping on, but is so rotten that a small stream of water 
allowed to impinge upon it makes a round hole. The collo- 
dion is non contractile, and so far from admitting of being 
pushed backward and forward after fixing with the hypo- 
sulphite, it at once breaks away in short pieces under such 
treatment. Fixing agents will often remove the image from a 
collodion of this kind, because the iodide of silver is not im- 
prisoned by the pyroxyline, but lies loosely upon its surface. 

Here we repeat the remark before made when speaking of 
the action of sulphuric acid, viz., that the secondary or dis- 
integrating action of nitric acid is always greater when the 
parchmentizing effect has been previously produced ; and we 
may also anticipate an observation to be made a few pages in 
advance, by adding that one kind of cellulose, such as flax, 
may be disintegrated by the nitric acid more readily than 
another, viz., cotton. 

To secure the full disintegrating action of weak nitric acid 
in the ordinary process of preparing pyroxyline, the bulk of 

* In addition to this modified pyroxyline produced by hot nitric acid 
mixed with a little sulphuric acid, a remarkable change of properties may 
be produced by the pure nitric acid of 1.45 employed cold, and without 
any admixture of sulphuric acid. The pyroxyline gradually becomes 
opaque, and loses its solubility in ether and alcohol ; eventually it dis- 
solves in the cold nitric acid without any evolution of gas, and if water be 
then added, opaque white flakes are thrown down, which, when treated 
with ether and alcohol, simply swell up without passing into solution. 



Pure Nitric Acid, 




1.45 at 60 deg. 


F. 


Water. 


. 1 




• 1 


. 1 




1 


. 1 . 




. A 


. 2 . 




• h 


. 3 . 




. 



134 THEORY OF PHOTOGRAPHY. 

nitric acid present in the nitro-sulpliuric acid should be at least 

equal to that of the sulphuric acid, and may with advantage 

be many times greater. Tlie cotton will then assume an 

opaque appearance on being di]3ped in the mixture, and the 

collodion will lack these properties of strength and toughness 

before referred to. 

Composition., hy Yolum^e, of Nitro-Sulphuric Acid for 

Prejpwring P kotograjjJdc, Pyroxyline. — The following table — 

Oil of Vitriol, 
1.815 at 60 deg. F. 

No. 1 . . . 3 . 

No. 2 ... 2 . 

No. 3 ... 1 . 

No. 4 ... 1 . 

No. 5 ... 1 . 

exhibits the composition of five different mixtures of sul- 
phuric and nitric acid, in which an attempt has been made to 
graduate the proportion of water in such a way that the per- 
centage of peroxide of nitrogen imparted to immersed cotton 
fibre may be nearly the same in each ; the table was con- 
structed by simple experiment, taking care in each case to 
work with the maximum quantity of water, and stopping the 
addition of water only when it was found that the product left 
a thick sediment on being dissolved in ether and alcohol. 

One hundred grains of purified and dried cotton immersed 
in either of these five mixtures, with the precautions neces- 
sary to prevent any loss from solution, ought to weigh one 
hundred and sixty grains ; the pyroxyline in each case is 
soluble, to a great extent, in glacial acetic acid, and also in 
boiling absolute alcohol ; whilst the resulting collodion does 
not produce an entirely opaque film (art. Pyroxyline). A 
more careful examination seems to indicate that if there be a 
difference in the strength of the mixtures given in the above 
table. No. 1 is somewhat stronger than No. 5.* The collo- 

* If any one should desire to examine the above acids as regards their 
strength or power of imparting NO:i to the fibre of cotton let it be borne in 
mind that in the case of No. 1 the percentage amount of NO3 in the pyroxy- 
line will increase considerably if the cotton be permitted to remain for 
several hours in the cold acid. The proportion of nitric acid in this mixture 
is so small, being little more than one-fifth of the total bulk, that the nitric acid 
is in a measure exhausted in acting upon the fibre, and consequently a 
rather low substitution body is formed ; the cotton imbibes the liquid like 
a sponge, and for the first half hour only those portions of the acid which 
touch the fibre produce any effect ; afterwards, however, a process of 
diffusion goes on in the liquid, and fresh acid coming into contact with the 
fibre, more peroxide of nitrogen is communicated to it. 



MANUFACTURE OF COLLODION. 135 

dion, however, from No. 1 is more fluid than that from No 5, 
thus showing that in addition to high temperature and dihition 
of the mixture with water, an excess of sulphuric acid has to do 
with flowing properties of collodion. A greater amount of 
fluidity than exists even in tlie collodion from No. 1 may be 
produced by dipping the pyroxyline flrst in No. 1, to secure 
the full action of the sulphuric acid, and afterwards in No. 
5 ; the weak nitric acid will then act more decidedly than it 
would have done upon a product produced by one immersion. 
The temperature employed for the above table of acids may 
be 150 deg. Fahr.; and, in making the pyroxyline, we And that 
the lower numbers give a product which has an opaque ap- 
pearance; whereas the pyroxyline made by Nos. 1 and 2 ex- 
hibits no opacity. The live samples of collodion difl'er very 
much in the rapidity with which they set upon the glass, and 
also in their physical structure ; the first setting rapidly and 
producing a horny film ; the last scarcely possessing any power 
of setting. The only way of overcoming this, and putting 
them on something like a par, is by varying the proportions of 
etiier and alcohol in the solvents, using more alcohol in the 
former, and more ether in the latter. Observe, however, that 
although the lower members of the series in the above table 
ought to yield a collodion setting less i-apidly upon the glass 
than the upper members, yet that this observation only applies 
when the tull quantity of water is enj ployed in the nitro-sul- 
phuric acid. For, as before shown, the setting power is in- 
jured by a weak nitric acid, but not so much by a stronger 
acid : hence No. 5 would produce a collodion with sufficient 
power of setting if the amount of water in the nitric acid were 
reduced ; but in this case the film would possess the objection- 
able properties of pyroxyline made in strong acids, being glu- 
tinous and difficult to pour. 

Affect of Raising the Temperature. — Although we have 
spoken of fluidity of collodion, and tenacity of the film as affected 
b}' the relative proportions of water, sulphuric acid, and nitric 
acid present in the nitro-sulphuric acid, yet such remarks must 
be taken very much in connection with the temperature at the 
time of putting in the cotton, since the physical modifications 
produced by the two constituent acids are seen in an exagger- 
ated degree when the nitro-sulphuric mixture is heated. An 
acid which gives an insoluble and explosive product in the 
cold, will yield pyroxyline perfectly soluble at a higher tem- 
perature. Pyroxyline prepared in acids barely warm, makes 
collodion, which is glutinous and difficult to pour on large 



136 THEORY OF PHOTOGRAPHY. 

glasses, even although containing as little as two or three grains 
of the soluble cotton to the ounce. The film soon becomes 
surface-dry, and repels the developer so that it cannot be made 
to flow up to the edge: on examination after fixing, it shows 
cellular spaces and structural lines. Hot acids, on the other 
hand, yield pyroxyline of the kind which is easily soluble in 
ether and alcohol to the extent of eight or ten grains to the 
ounce of solvents, and the resulting collodion is limpid and ad- 
herent to the glass. There is also an absence of structural 
marking in this collodion, the transparent layer being nearly 
homogeneous even when highly magnified. 

The reader will perceive that the above physical efiects of 
increased temperature are almost identical with those before 
attributed to a dilution of the nitro-sulphuric acid with water. 
This is quite natural, because heat and dilution co-operate in 
increasing the action of the acids upon the fibre. What we 
have to do in preparing a pyroxyline for fluid and adhesive 
collodion, is to hit the exact point at which disintegration of 
the fibre commences, and to add more water, and raise the 
temperature a few degrees, if after an immersion of five or ten 
minutes the pyroxyline appears strong and unyielding like or- 
dinary cotton. 

Different forms of Cellulose. — Cotton, straw, pith, flax, 
etc., with the mannfactured fabrics produced from the same, 
may be converted into soluble pyroxyline, but the product 
will be more or less different in each case. When a rather 
concentrated nitro-sulphuric acid is used, cotton may give a 
glutinous collodion, and calico, a fluid collodion. In another 
acid, weaker than the last, the cotton succeeds well, whilst 
calico instantly dissolves. The difference in the two cases ap- 
pears to depend principally upon the thickness of the fibre ; 
caKco produces pyroxyline of the fiuid kind, and is partially 
dissolved because the nitric acid, in acting on the outside por- 
tions of the closely-twisted fibre, is reduced in strength, and 
hence the interior of the fibre is left more nearly in the condi- 
tion of xyloidine. It is possible, also, that with this weaken- 
ing of the acid there may be a corresponding rise of temper- 
ature, which would assist in producing a powderly pyroxyline 
or in causing solution. 

Linen, even when selected of the same thickness, yields a 
more limpid collodion than calico, and one of which the film 
is less tenacious and contractile. These peculiarities cannot be 
satisfactorily explained, unless they depend upon a difference in 
composition; which is not improbable, since it is known that 



MANUFACTURE OF COLLODION. 137 

flax can be distinguished i'rom cotton by chemical tests, being 
more easily discolored by the action of alkalies. 

Both linen and calico undergo a change by constant use, 
which is recognized on making the material into pyroxyline. 
Old and rotten rags are quickly disintegrated by the acid mix- 
ture and the pyroxyline produces a highly structureless collo- 
dion, which adheres with much tenacity to the glass. Paper 
has been at different times much recommended for the prepa- 
ration of pyroxyline, but it will easily be gathered from the 
above remarks that it is an unfit material ; for not only do dif- 
ferent samples of paper vary greatly in thickness, which, as be- 
fore shown, would affect the action of the nitric acid, but, 
being made both from cotton and linen rags, some of which 
rags are new, and others old and rotten, they cannot be ex- 
pected to produce uniformity. The Swedish Altering paper 
imported into this country for chemical purposes, has been 
stated to be uniform, but this assertion is not altogether cor- 
rect ; the pyroxyline being at one time structureless, and at 
another comparatively glutinous even with the same acids. 

In addition to the materials mentioned above, others have 
been tried, such as China grass, the pith of the Jerusalem ar- 
tichoke, the fibre of the aloe, etc., but the result was only to 
confirm the opinion above expressed, that on each material 
the sulphuric acid, and also the nitric acid, produces a differ- 
ent effect. Tlie fibre of the grasses, including flax, appears 
to be more easily disintegrated and dissolved both by acids and 
alkalies than that of cotton, and to be convertible into sugar 
with greater facility. 

A point of some importance as regards the manufacture of 
pyroxyline, is the cleansing of the fibre thoroughly from ad- 
hering resinous matter, which, if allowed to remain, deoxidizes 
a portion of the nitric acid, and so far weakens it as to insure 
the immediate destruction of a portion of the cellulose at high 
temperatures. A convenient substance to employ in cleans- 
ing is a diluted alkali, which converts the resin into a soap 
more or less soluble in water. It is probable that the differ- 
ences which have been said to exist between cotton of various 
growths may depend in part upon the presence or absence of 
this resinous matter. The manufacturer who wishes to work 
with great accuracy, and to employ the largest quantity of 
water possible in the nitro-snlphuric acid, should also bear in 
mind that cotton is a hygroscopic substance, and requires to 
be artificially dried. A minute proportion of moisture present 
upon the very surface of the cotton, would produce a greater 



138 THEORY OF PHOTOGRAPHY. 

effect in causing solution than the same quantity of water ad- 
ded to the nitro-sulphuric acid, since it would dilute only that 
portion of the acid which touches the iibre, and thus would 
cause a rise in the temperature. 

Photographic Properties of Various Kinds of Pyroxyline. 
— In preparing pyroxyline from the five different mixtures 
given in the table of composition of nitro sulphuric acid at 
page 155, it is found to differ in its photographic properties as 
well as in its physical properties. As regards intensity of the 
negative image, we observe that pyroxyline made in the mix- 
tures at the top of the scale, which contain an excess of the 
diluted oil of vitriol, tends to produce more intense images 
than pyroxyline from ISIo. 4 or No. 5, in which the diluted 
nitric acid is in excess. The difference in the two cases is 
caused by the oil of vitriol, since the same effect of increasing 
intensity can be obtained by parchmentizing the fibre first, and 
converting it into pyroxyline afterwards ; and it is not im- 
probable that this action of the sulphuric acid in increasing in- 
tensity depends upon a conversion of the cellulose into a sub- 
stance resembling dextrine in its photographic action. 

Irrespective of the proportions of the two acids, the quantity 
of water in any mixture of nitro-sulphuric acid will affect the 
photographic intensity of the resulting pyroxyline and col- 
lodion. The effect is evidently due to the same action of 
weak nitric acid as that which causes it to disintegrate the fibre, 
for if we take a sample of pyroxyline previously parchment- 
ized by oil of vitriol, and capable of yielding great intensity in 
collodion, we may destroy its properties in that respect most 
completely by dipping it for an instant in a warm mixture 
containing an excess of very weak nitric acid. When speaking 
of the negative nitrate bath, it will be shown that it loses its 
power of producing a dense picture if a little organic matter 
oxidized by nitric acid be added to it ; it appears, therefore, 
that nitrate acid is capable of producing with organic bodies a 
substance of unknown composition, which is injurious to the 
intensity of the photographic image. This subject, however, 
is not at present clearly understood, and we must, therefore, 
be satisfied with indicating the facts as thej stand. 

The temperature of the nitro-sulphuric acid at the time of 
immersing the cotton invariably affects the photographic prop- 
erties of pyroxyline. At high temperatures, a portion of the 
fibre is converted into a substance which has a bitter taste, and 
turns brown when treated with alkahes. This substance is be- 
lieved to be nitro-glucose, formed by the action of strong nitric 



MANUFACTURE OF COLLODION. 139 

acid upon grape sugar ; the grape sugar itself being produced 
from the cellulose by contact with the warm and diluted sul- 
phuric acid. In studying the effect which this bitter product of 
decomposition is likely to produce, we may prepare nitro- 
glucose, and add it to collodion. Nitro-glucose diminishes the 
sensitiveness of the film to weak rays of light, but increases the 
rapidity and intensity of tlie development in negative pictures. 
Vigorous images are produced even in a very dull light, but 
they are always liable to be black and white without middle 
tints, or to solarize, and become extremely red in the most ex- 
posed parts of the film, where the light acts strongly. Collodion 
made at very high temperatures, although possessed of great 
fluidity, adhesiveness, and freedom from structural lines, with 
other physical advantages, before enumerated, is found less 
useful for ordinary work than samples prepared at a lower 
temperature, and which are not so good in physical properties, 
being somewhat ropy in hot weather, and drying up more 
quickly after sensitizing. A film of this latter kind is very 
sensitive, and every radiation makes a distinct impression even 
after the shortest exposure. 

The author of this work is inclined to attribute the peculiari- 
ties of collodion made from linen, or from paper manufactured 
out of partially decomposed rags, to the presence of nitro- 
glucose in the pyroxyline. lie has found that unless the tem- 
perature of the nitro-sulphuric acid be kept low, there is a 
peculiar disposition to form the bitter resin, not only in linen 
fibre, and partially decayed cotton fibre, but also in the pure 
cotton fibre previously converted into vegetable parchment by 
the action of diluted sulphuric acid. In each case the collodion 
is highly intense, and when shaken up with carbonate of potash 
assumes an amber-yellow color, whereas a pyroxyline nearly 
free from the bitter matter remains colorless for a time on 
treating the collodion with carbonate of potash. 

It may be proper before leaving this part of our subject to 
say a few words on an action with chlorine appears to exert in 
the manufacture of pyroxyline. The yellow nitric acid of 
commerce invariably contains a portion of chlorine, and this is 
found to exert a decomposing action upon the fibre, the result 
•of which is to increase the fluidity of the resulting collodion, 
and also its intensity, but somewhat to diminish its sensitive- 
ness. It is advisable therefore to employ a nitric acid from 
which the chlorine has been eliminated, since any amount of 
intensity of collodion may be obtained by sufficiently increasing 
the proportion of diluted oil @f vitriol in the nitro-sulphuric 



140 THEORY OF PHOTOGEAPHY. 

acid, or by raising the temperature, and avoiding tlie use of 
too mucli water in the acids. 

In examining commercial samples of pyroxyline prepared 
from cotton-wool, the writer has met with a variety which 
gives great intensity of collodion with an average amount of 
fluidity. A quantity of pyroxyline similar to this may be 
prepared in a mixture of nitro-sulphuric acid containing about 
two measures of oil of vitriol to one measure of nitric acid of 
1.45 (contaminated with a little chlorine), and a quantity of 
water decidedly less than that given in the table at page 134. 
The temperature of the acid mixture must be sufficiently 
raised to disintegrate the fibre, and so to produce a pyroxyline 
which occupies a small space when dry, has rather a yellow 
aspect, and is inclined to be dusty. The film from the result- 
ing collodion is not very sensitive, since it contains a notable 
portion of the bitter resin, but it answers remarkably well for 
a negative collodion prepared with a mixed iodide and bro- 
mide, and its manufacture requires less nicety than the form- 
ula which the author adopts, since the quantity of water in 
the acids is not so large. 

Spontaneous Decomposition in Pyroxyline. — The author ha& 
occasionally failed when using samples of pyroxyline which 
have been kept for many months after preparation. A par- 
tial liberation of oxides of nitrogen appears to take place in 
some instances, forming an atmosphere of red fumes within 
the bottle. Pyroxyline which has undergone much decompo- 
sition from the use of a very high temperature in the process 
of manufacture may be expected to change in this way, and 
especially so if the acids are not thoroughly removed by 
washing; a little sulphuric acid left in the pyroxyline would 
keep it continually damp, and perfect dryness is essential to- 
the stability of pyroxyline. From phenomena observed in 
the decomposition of nitro-glucose, it is probable that constant 
exposure to light favors the change. The products of the 
spontaneous decomposition of gun cotton appear to be the 
oxallic acid and a neutral organic substance having the com- 
position of gum. 

THE PLAIN COLLODION. 

The relative proportions of ether and alcohol in collodion 
affect both the physical and photographic properties of the 
solution. This subject has been already alluded to at page 134^ 
but there are some additional observations to be made. 



PLAIN COLLODION. 141 

The use of an excess of alcoliol is advantageous in prevent- 
ing the ether from evaporating quickly in hot weather. At 
temperatures of 90 deg. and 100 deg. Fahr., it is almost im- 
possible, with collodion containing but little alcohol, to coat a 
large glass and immerse it in the bath before the upper part 
becomes dry ; consequently an uneven coating of iodide is 
produced, with a blueness at the top edge. This happens 
especially when the spirits are very strong and almost free 
from water. Another advantage of making collodion with a 
large quantity of alcohol is, that it is more readily wetted by 
the bath solution, and does not throw the liquid into greasy 
lines upon the surface of the film. There is, however, a prac- 
tical limit to the use of alcohol, inasmuch as not being a com- 
plete solvent of pyroxyline, it alters the structure of the film, 
rendering it gelatinous. 

The quality of the pyroxyline is the first important point to 
be considered in determining the relative proportions of ether 
and alcohol. With a sample of pyroxyline made at a low 
temperature and in rather strong nitro-sulphuric acid contain- 
ing a minimum of sulphuric acid, barely enough alcohol 
should be used in the plain collodion to confer the requisite 
solubility, viz., an eighth or a twelfth part, by bulk, of the 
ether; otherwise the iodized''^ collodion will be very tender 
and easily torn, glutinous and difiicult to pour, loosely ad- 
herent to the glass, and full of crapy lines and structural 
markings. 

Pyroxyline prepared from cotton-wool by the formula with 
a large excess of oil of vitriol, will bear more alcohol than the 
last, and with some advantage, for the contractility which 
makes it separate from the edges of the glass, is lessened by 
the addition of alcohol. The tendency to set quickly and pro- 
duce water-markings at the upper edge of the plate is also to a 
treat extent obviated, as is also the rapid surface-drying of the 
Im after taking it from the bath, which causes it to repel the 
developer, as before shown. The film being nearly structure- 
less and very tough, will bear a quantity of spirits, which, in 
other cases, would produce crapy lines and tenderness. If the 
pyroxyline of this formula be made in acids containing the 
largest possible quantity of water, the proportion of alcohol in 
the plain collodion may be one-half of the ether, which will 
give, after iodizing, equal bulks of the two solvents ; but if the 

*This observation supposes the plain collodion and the iodizing com- 
pound to be kept in separate solutions ; two measures of the latter being 
added to six measures of the former. 



142 THEORY OF PHOTOGKAPHY. 

nitro-sulphuric acid was made with less water, then the pro- 
portion of alcohol in the plain collodion must not exceed one- 
third of the ether, or the film will be woolly at the lower cor- 
ner of the plate. 

It will be fonnd that the solubility of this tough kind of 
pyroxyline is increased by employing the maximum quantity 
of alcohol, so that if the plain collodion be diluted with ether a 
precipitate will take place. With other kinds of pyroxyline 
differently prepared, the addition of ether to the plain collodion 
produces no precipitate. 

In the case of pyroxyline prepared in nitro-sulphuric acid, 
containing equal bulks of oil of vitriol and nitric acid, with 
the maximum of water, it is advisable to reduce the quantity 
of alcohol somewhat ; for if too much alcohol be employed, 
the setting of the pyroxyline will be so greatly retarded that 
the upper edge of the film will become dry before the lower 
part has solidified sufficiently to take the bath without pre- 
cipitation of the pyroxyline. Su(.'h an effect could not happen 
in the case of the first formula of page 134, containing oil of 
vitriol in excess, because it would be impossible to use such a 
mixture in a state sufficiently weak to destroy the property of 
setting in the resulting pyroxyline ; before that point was 
reached the cotton would dissolve in the acid. 

The exact strength of the alcohol used in photography must 
always be noted, since the effect of water when present in any 
quantity is to produce viscidity of collodion, and more rapid 
decomposition under the influence of the iodizing compound. 
The experience of photographers is favorable to the employ- 
ment of even a stronger spirit than that usually recommended ; 
and when it can be obtained the alcohol of .805 at 60 deg. 
Fahr., sold in commerce as absolute, may be preferred as a 
rule. 

With the most horny kind of pyroxyline, however, prepared 
by the first formula of page 134, a little water appears^ neces- 
sary, to open out the structure of the film, and prevent it from 
assuming a condition in which it resembles gutta percha in 
being impervious to liquids; but in the case of pyroxyline 
from formula No. 3, of page 134, such as is recommended from 
positives, the film will be sufficiently porous, even with the 
whole of the alcohol in the absolute state. With formula No. 
1, the alcohol of .805 may also be employed, if the proportion 
be increased until it nearly doubles that of the ether. 

Photographic Effects of Excess of Alcohol in Collodion. — 
The addition of alcohol to collodion lessens the contractility of 



PLAIN COLLODION. 143 

the film, and renders it soft and gelatinous. These conditions 
are favorable to sensitiveness, perhaps from the play of affin- 
ities being promoted by the loose manner in which the particles 
of iodide are held together. The extra sensitiveness obtained 
by use of alcohol, however, does not increase after a certain 
point ; on the contrary, it diminishes, for it appears to be ne- 
cessary to extreme sensitiveness that the film should coagulate 
within a certain time after it has been coated, and therefore 
the addition of alcohol must be stopped when the film loses its 
ready setting qualities, and is not coherent under the finger. 
Hence a porous collodion is soon injured in sensitiveness on 
adding too much alcohol, but a strong and tough pyroxyline 
will bear equal bulks of the two solvents without loss in that 
respect. 

The above observation as to the effect of excess of alcohol in 
diminishing the sensitiveness of the film to dark objects ap- 
plies particularly when the atmosphere is cold and damp, and 
evaporation is retarded. At a very high temperature, and in a 
dry air, it does not apply, since the presence of alcohol is then 
useful in preventing the film from becoming surface-dry, espe- 
cially when it is necessary to keep the sensitive plate for a long 
time between exposure and development 

Intensity of negative is much favored by using a full quan- 
tity of alcohol, and particularly when large glasses are coated, 
and long focus lenses, which work slowly, are employed. The 
difference is doubtless due in part to the structure of the film 
being opened out by the alcohol, so as to assist the developer 
in penetrating, and partly to the rapidity of evaporation and 
consequent surface-drying being diminished. Hence in hot 
weather the alcohol acts very beneficially, and with collodion 
made almost entirely of ether, the negatives are unusually 
weak at such times. Pyroxyline of the horny kind is 
especially liable to lose intensity at high temperature, unless 
the alcohol be added freely so as to prevent the film from be- 
coming hard and impervious. 

Decomposition of Plain Collodion hy Keeping. — Plain col- 
lodion tends to become more liquid by keeping, and often ac- 
quires the property of eliminating iodine rapidly from the 
iodizer; but the rapidity with which the change takes place, 
varies much with the mode of preparing the pyroxyline, and 
with the quality of the ether. Supposing all the materials to 
be pure, the decomposition, after keeping for several months 
in a cool and dark place, is very slight, and is generally con- 
sidered to improve the quahty of the negative rather than 



144 THEORY OF PHOTOGRAPHY. 

otherwise ; it imparts a slightly red or purple tone to the image, 
without much affecting the sensitiveness, and hence many pre- 
fer to keep a stock of collodion always on hand, tliat it may 
settle down clear, and " ripen." This proceeding, however, 
would be very far from safe with an unstable pyroxyline, or an 
inferior ether, since the oxides of nitrogen would then be set 
free from the gun cotton, and injury to the sensitiveness would 
result from elimination of iodine after iodizing. 

When great stability is an object, as in exporting collodion to 
foreign climates, the pyroxyline ought not to be made in such 
a way as to produce m.uch decomposition in the nitro sulphuric 
acid. For instance, if we take pieces of old linen and immerse 
them at a temperature of 150 deg. Fahr., in a nitro-sulphuric 
acid made purposely very weak, the greater part will dissolve, 
but a few fragments remain, which, when washed and applied 
to the tongue, have a bitter taste. In this case the pyroxyline 
is partially decomposed in the acid, and collodion made from 
the product might work well at first, but after keeping for 
twelvemonths in the plain state would probably be as thin as 
water, and produce a rotten and insensitive film. 

The best kind of pyroxyline for yielding a stable collodion, 
according to the author's experience, is that made from cotton- 
wool, and at rather a low temperature. This preparation has 
been proved to retain its original properties nearly unchanged 
for nine months. The test for decomposition of pyroxyline is 
agitation of the collodion with dry carbonate of potash. When 
so treated it should remain colorless for a certain time ; if it 
assume a brown tint in less than two hours, traces of the com- 
pound above named as resembling nitro-glucose are present. 

Plain collodion, made at a temperature of 170 to 180 deg. 
Fahr., wall keep well for a few weeks, but when exported to a 
distant climate, and subjected to an elevated temperature, it 
liberates iodine from iodide of potassium somewhat quickly, 
and is useful only for taking positives, or negative views, when 
shortness of exposure is not an object. Carbonate of potash 
shaken up with this kind of collodion strikes a brown color. 

A point which should not be overlooked in preparing col- 
lodion for keeping is the length of time during which the fibre 
of the cellulose remains in the nitro-sulphuric acid. A short 
immersion (five minutes) is the best, but if the material be 
left for half an hour or longer, as may sometimes be done 
without solution taking place, the collodion will often acquire 
by degrees the property of striking a brown color with iodide 
of potassium. 



IODIZING SOLUTION. 145 

The quality of the ether is very important in making a 
collodion of uniform properties. It is possible to obtain an 
ether which will remain for months without assuming the con- 
dition known as " ozonized." in which it liberates iodine from 
iodide of potassium ; but ether containing aldehyde or organic 
impurity will soon change. 

The photographer will be guided partly by the quality of 
his ether in deciding as to the best fornmla for a pyroxyline, 
because an inferior ether soon renders the collodion rather 
limpid through formation of acetic ether or some similar pro- 
duct, by which the setting power of the pyroxyline is di- 
minished. It must also be borne in mind that when this im- 
pure ether liberates iodine in the iodized collodion, a corres- 
ponding portion of the alkali potash is set free, which, as we 
shall presently show, adds to the limpidity, and lessens the set- 
ting powers. To meet this difficulty, the pyroxyline should 
be made slightly more tough and contractile than is necessary, 
which may be eifected by reducing the temperature of the 
acid a few degrees. Also, since impure ether soon becomes 
ozonized, a high temperature of nitro- sulphuric acid ought to 
be avoided, as likely to render the pyroxyline unstable. 

Inferior ether usually produces a more intense collodion 
than pure ether, but the plain collodion does not keep for any 
length of time without change, soon losing its sensitiveness, and 
yielding pictures which are black and white without middle 
tints. A rapid elimination of iodine takes place on adding the 
iodizer, and the film is weak and rotten. In some cases, com- 
mercial ether is contaminated with organic oils having a foul 
smell, or contains traces of the alkali employed in the process of 
purification. The latter impurity is particularly objectionable, 
because alkalies and carbonated alkalies decompose collodion, 
rendering it limpid, and destroying the setting properties. 
" Methylated Ether " (see the vocabulary), is largely employed 
in the manufacture of collodion, but where expense is not an 
object, the writer recommends the pure ether in preference. 
(Head the article " Ether," in the vocabulary, for further par- 
ticulars.) 

IODIZING SOLUTION. 

Different iodides vary in their effect upon plain collodion. 
Those which have an acid reaction, like the iodide of cadmium, 
may be expected to increase the glutinosity. But alkaline 
iodides, such as iodide of potassium or ammonium, render it 



146 THEORY OF PHOTOGRAPHY. 

limpid and structureless. Fixed alkalies or alkaline carbon- 
ates have a more marked effect than alkaline iodides, as may 
be shown by agitating plain collodion with powdered carbonate 
of potash ; in the course of a few days it becomes as fluid as 
water. The first effect of the alkali, however, upon pyroxy- 
line, made by the formula with excess of oil of vitriol, is to 
render the collodion glairy, so that the bottle may often be 
completely inverted without any immediate loss. The same 
observation applies to alkaline iodides ; they thicken the col- 
lodion slightly in the ffrst instance, and afterwards render it 
more limpid. When collodion gradually becomes limpid 
under the action of an alkaline iodide, it loses its power of set- 
ting upon the glass ; but there is a remarkable difference be- 
tween collodions in the rapidity of this change, and those which 
are tough and unyielding when newly made withstand the 
action of the alkali for a much longer time. 

The reader will perceive that the effects now attributed to 
the action of alkaline iodides upon collodion, viz., limpidity 
and a diminished power of setting, are the same as those spoken 
of under the head of dilution of the nitro-sulphuric acid with 
water. There is, however, this important difference in the two 
cases, that in the former an increase of photographic intensity 
accompanies the porosity, but in the latter the intensity is 
diminished, as already shown. 

Iodide of Cadmium. — The stability of this compound is its 
great recommendation ; ether, unless highly ozonized, has 
little or no effect in liberating iodine from it. Another ad- 
vantage of the iodide of cadmium is that it does not destroy 
the setting properties of the pyroxyline as the alkaline iodides 
eventually do. 

The collodion should be prepared purposely when iodide of 
cadmium is used as an iodizer. Collodion containing pyroxy- 
line made at a low temperature, and in rather strong acids, 
works tolerably well after iodizing with the potassium or am- 
monium compound, and keeping until liberation of iodine and 
liquefaction have taken place ; but with iodide of cadmium it 
would be in every respect unsatisfactory, flowing in a slimy 
manner upon the plate, exhibiting crapy lines, repelling the 
developer, and splitting away from the glass on drying. When 
the pyroxyline is made in strong acids at a very high tempera- 
ture, the collodion is often sufficiently fluid, but yet is not al- 
together adapted for iodizing with the cadmium salt, from its 
tendency to produce " woolliness" or uneveness of fllm at the 
lower edge of the plate. The proper kind is that prepared in 



IODIZING SOLUTION. 147 

weak acids, and sufficiently parchmentized by the oil of vitriol 
to give the requisite degree of intensity. 

Iodide of Potassium. — This salt is well adapted for iodizing 
collodion not required to possess keeping properties, but its 
sparing solubility in alcohol and ether is an objection. Col- 
lodion containing 4|- parts of ether of .725 to 3|- of alcohol 
,816, will carry nearly three grains of iodide of potassium to 
the ounce. With equal bulks of ether and alcohol the full 
quantity, viz., four grains, may be dissolved. Five parts of 
ether with three of alcohol will not take more than 2 or 2^ 
grains to the ounce. The admixture of iodide of cad- 
mium with iodide of potassium increases the solubility of 
the latter salt, by forming a double salt, the iodide of potas- 
sium and cadmium. To produce this compound, wliich is well 
fitted for iodizing collodion, containing five parts of ether to 
three of alcohol of .816, equal weights of the two iodides may 
be taken. These proportions are not strictly correct, but they 
are sufficiently near for practical purposes. 

It may also be remarked that the quantity of an alkaline 
iodide which any collodion will carry depends partly upon the 
pyroxyline. With that particular kind of pyroxyline which is 
recommended for negatives in this work, no more than 3-g- 
grains per ounce must be employed, otherwise the iodide of 
silver will be precipitated upon the surface of the film at the 
lower edge, and marks will result. By mixing the iodide of 
potassium with iodide of cadmium, or by using iodide of cad- 
mium alone, a larger quantity of iodide may be used without 
the appearance of markings. 

The effect of using an iodizing solution containing more 
iodide of potassium than the collodion will retain in solution, 
is not always to produce an immediate precipitation, such as 
would result if the pyroxyline were omitted, and the iodizing 
solution added to an equivalent quantity of ether and alcohol. 
The presence of the pyroxyline prevents a visible deposition, 
but the collodion will produce a spotted image, and after 
standing for some days, crystals will form upon the sides of 
the bottle. 

When the whole of the alcohol contained in the collodion is 
of the strength of .805, commercially known as "absolute," it 
will scarcely be safe to iodize with iodide of potassium only, 
unless the proportion of alcohol is nearly double that of the 
ether ; much, however, will depend upon the degree of dry- 
ness of the ether itself, since the commercial ether often con- 
tains water dissolved in it, in sufficient quantity to prevent the 



14:8 THEORY OF PHOTOGRAPHY. 

precipitation of the iodide of potassium. The preparation of 
an iodizing solution of iodide of potassium is a troublesome 
process with alcohol of .805, since it is necessary to pulverize 
the iodide very carefully, and to boil the spirit upon it ; in 
cold weather such an iodizer is apt to deposit cubical crystals 
upon the sides of the bottle containing it. 

Commercial iodide of potassium is often contaminated with 
carbonate of potash. This salt has an injurious action, not 
only in throwing a white deposit of carbonate of cadmium 
when the mixed iodides are used, but also in reacting upon the 
collodion, and producing rapid liquefaction, as before shown. 
Pure iodide of potassium is now prepared purposely for pho- 
tography ; the crystals have a slightly yellow tint, and the 
alcoholic solution liberates iodine slowly on exposure to 
light. 

Iodide of ammonium is useful in iodizing collodion, when it 
is required to add also a portion of bromide ; if any iodide of 
potassium were present in such a case, a white deposit would 
form, on account of the sparing solubility of bromide of potas- 
sium in spirits free from water. Collodion containing 4^ 
drams of ether to 3^ of alcohol .816, will not carry more than 
a quarter of a grain of bromide of potassium to the ounce, but 
it will easily dissolve a much larger quantity of bromide of 
ammonium or bromide of cadmium. The only objection to 
the use of iodide of ammonium is its instability, and the diffi- 
culty of invariably obtaining it in a pure state, the writer hav- 
ing found that most of the samples prepared by aid of 
sulphuretted hydrogen, or hydrosulphite of ammonia, contain 
traces of a sulphur compound, and are inferior to those made 
in the moist way, by precipitation. When iodide of am- 
monium becomes brown by keeping, it may be decolorized by 
shaking it up in a bottle with a little ether, and drying upon 
blotting paper. 

Iodide of sodium is intermediate in solubility between 
iodide of potassium and and iodide of ammonium, and would 
be a valuable compound for iodizing if it could be obtained 
commercially in a pure state. 

Iodide of iron was formerly used in photography, but has 
now become obsolete. It produces a very sensitive collodion 
at first, but soon reacts upon the pyroxyline, and the collodion 
becomes reduced to the condition of a jelly. The nitrate bath 
is also thrown out of order, protonitrate of iron being 
formed, which precipitates metallic silver on the sides of the 
trough. 



IODIZING SOLUTION. 149 

Chemical and Photographic Action of the Various Iodides 
in Collodion. — With recently iodized collodion the difference 
in sensitiveness between the various iodides is not very marked 
if they are in a pure state. The presence of carbonate of pot- 
ash, iodate of potash, or chloride of potassium in the commer- 
cial iodide of potassium at once diminishes the sensitiveness ; 
and any trace of a sulphur compound in the iodide of am- 
monium will have the same effect. 

The intensity of negative collodion does not vary ma- 
terially with the iodide, if the collodion be tested soon after 
iodizing. Nevertheless, by close observation, minute differ- 
ences can be detected, and iodide of potassium will be found 
to give a somewhat more vigorous picture than iodide of am- 
monium. With any iodide also the intensity will be lessened 
by adding too large a quantity of the iodide, and especially 
when the setting power of the collodion is small. If, from 
any cause, such as excess of iodide, deficiency of setting power 
in the collodion etc., the sensitive film of iodide be allowed to 
lie loosely upon the surface of the collodion, the picture will 
be very feeble, and will often fall away when the fixing agents 
is applied. 

After collodion has been kept for a time in the iodized state, 
both the sensitiveness and the intensity will vary with the 
iodide chosen, because, as before shown, an affinity exists be 
tween the pyroxyline and the base of the iodides. Alkaline 
iodides are soon decomposed by collodion, and hence a loss of 
sensitiveness, depending partly upon the retarding effect of 
free iodine, and partly upon the new compound formed by the 
liberated alkali. The liquefaction of the collodion, and its 
diminished power of setting upon the glass, must also be sup- 
posed to be injurious to sensitiveness. The intensity, how- 
ever, will increase in consequence of these changes, since free 
iodine, although it lessens intensity in a dull light, tends to 
prevent feebleness of the image from solarization in a strong 
light ; and the compound formed by the trace of the liberated 
alkali, whatever be its nature, has a direct action in adding to 
the intensity. The liquefaction of the collodion is also service- 
able up to a certain point, by removing the impermeability to 
the developer which is the more dense and horny kinds of col- 
lodion exhibit. 

Iodide of cadmium is the most stable of all the iodides, yet 
with iodide of cadmium as an iodizer, an amount of change 
sufficient to lessen the sensitiveness of the collodion to very 
dark objects eventually takes place, and with an unstable 



150 THEORY OF PHOTOGKAPHY. 

pjxoyline or impure ether, tliis change may be evident even 
in a few weeks. Iodide of cadmium also increases the inten- 
sity of collodion after a long keeping, especially with pyroxy- 
line made in rather strong acids. In such a case the collodion 
gradually becomes gelatinous, the iilm solarizing strongly in 
the parts most acted on by the light, and the image exhibiting 
a bright red color, when viewed by transmission. This 
state of collodion is well fitted for photographing in a dull light. 

The nature of the pyroxyline must also be taken into account 
in estimating the probable effect of iodizing with an alkaline 
iodide. If it has undergone decomposition in the manufacture, 
the sensitiveness of the collodion will soon be lost ; but if pre- 
pared at low temperatures, it remains for a long time un- 
changed ; of all kinds of pyroxyline the least stable after 
iodizing is that prepared from linen in weak acids and at a 
high temperature. 

A point which aifects the keeping qualities of a collodion, 
r its stability after iodizing, is- the presence of bromide com- 
bined with the iodide. Collodion so made assumes the yellow 
tint when mixed with the iodizing solution ; but in the course 
of some hours either the whole or a part of the free iodine ap- 
pears to be re-absorbed. This change takes place most rapidly 
in the case of methylated ether, but it may be seen more or 
less even with pure ether and pure alcohol. 'No satisfactory 
explanation can be given. 

To facilitate the comprehension of the decompositions which 
take place in iodized collodion, let the following experiments 
be made. Take nitro-glucose and add it in small quantity to 
ordinary collodion iodized with the potassium compound ; the 
elimination of iodine will be more rapid than usual, the col- 
lodion at the same time losing sensitiveness and gaining in- 
tensity. Next dissolve nitro-glucose in spirits of wine, and boil 
it in a test-tube with powdered carbonate of potash ; the liquid 
becomes brown, and evolves a smell of burnt sugar ; a few drops 
of it in iodized collodion rapidly destroys the sensitiveness, 
but adds much to the blackness of the negative. In a third ex- 
periment, introduce a portion of a reducing agent, such as 
grape-sugar into iodized collodion ; the result will be to lessen 
the sensitiveness on keeping, and to increase the intensity. 
The experiment last described shows that not only the nature 
of the particular iodide, and that of the pyroxyline, but also 
the presence of foreign organic substances may affect the 
keeping properties of iodized collodion ; hence the importance 
of using ether which has been carefully freed from traces of 



IODIZING SOLUTION. 



151 



aldehyde, etc., since even the employment of an iodide as 
stable as that of cadmium will not prevent decomposition if 
substances are present in the collodion which have an affinity 
for oxygen. 

Another experiment illustating the effect of changes in col- 
lodion, after iodizing, is the following : Take nitrate of soda, 
and boil it with alcohol of .805, until a saturated solution has 
been obtained ; a few minims of the liquid, added to an ounce 
of iodized collodion, will increase the intensity. A nitrate is 
one of tlie products of decomposition of collodion by alkalies 
or alkaline iodides, and may sometimes be seen in the form of 
well-defined crystals at the bottom of the bottle. 




CHAPTER YII. 

THE CHEMISTRY OF THE NITRATE BATH. 

The solution of nitrate of silver in which the plate coated 
with iodized collodion is dipped to form the layer of iodide of 
silver, is known technically as the Kitrate Bath. The chem- 
istry of nitrate of silver has been explained at page 96, but 
there are some points relating to the properties of its aqueous 
solution which require a further notice. 

Solubility of Iodide of Silver in the Nitrate Bath. — Aque- 
ous solution of nitrate of silver may be mentioned in the list 
of solvents of iodide of silver. The proportion dissolved is in 
all cases small, but it increases with the strength of the solution. 
If no attention were paid to this point, and the precaution of 
previously saturating the nitrate bath with iodide of silver 
neglected, iodide of silver in the film would be dissolved when 
left too long in the liquid. 

This solvent power of nitrate of silver on the iodide is well 
shown by taking the excited collodion plate out of the bath, 
and allowing it to dry spontaneously. The layer of nitrate on 
the surface, becoming concentrated by evaporation, dissolves 
the iodide and produces a transparent, spotted appearance. 

In the solution of iodide of silver by nitrate of silver, a 
double salt is formed, which corresponds in properties to the 
double iodide of potassium and silver in being decomposed by 
the addition of water. Consequently, in order to saturate a 
bath with iodide of silver it is only necessary to dissolve the 
total weight of nitrate of silver in a small bulk of water, and 
to add to it a few grains of an iodide ; perfect solution takes 
place, and on subsequent dilution with the full amount of 
water, the excess of iodide of silver is precipitated in the form 
of a milky deposit. 

The above-named double salt of iodide of silver and nitrate 
of silver has been termed by Schnauss the " iodo-nitrate of sil- 
ver ;" and although in the collodion nitrate bath it is present 
only in small quantities, and in a state of solution, it is quite 
possible to obtain it in well-defined crystals. No correspond- 
ing compounds containing bromide and chloride of silver are 



THE NITRATE BATH. 153 

known, and hence it will not be necessary to satnrate the 
bath with those salts when it is desired to use them in col- 
lodion. 

Although the addition of water to the negative bath renders 
it milky by precipitation of iodide of silver, dilution with al- 
cohol has not the same effect. 

Acid condition of Nitrate of Silver. — A solution of nitrate 
of silver prepared from the commercial nitrate has usually an 
acid reaction ; the crystals having been imperfectly drained 
from the acid mother-liquor in which they were formed. 
Hence, in making a new bath, it is advisable not only to satur- 
ate it with iodide of silver, but to neutralize the free nitric 
acid it contains. 

The quantity of this acid is very variable. If the niti-ate 
has been carefully dried at 240 deg. Fahr., and then crystal- 
lized a second time, no nitric acid can be detected, the concen- 
trated aqueous solution slowly restoring the blue color of red- 
dened litmus. This alkaline effect upon reddened litmus, the 
writer believes to be the proper reaction of pure nitrate of sil- 
ver, since he finds it to exist in samples of nitrate which have 
never undergone fusion. On the other hand, nitrate of silver, 
, which has been crystallized only once from the acid mother- 
liqnor, without any attempt at carefnl drying, is often so de- 
cidedly acid that it cannot be employed for the bath until 
neutralized. 

The nitrate bath, although perfectly neutral when first pre- 
pared, may become acid by continued use, if collodion contain- 
ing much free iodine be constantly employed. In that case a 
portion of nitric acid is liberated, and iodate and iodide of sil- 
ver are forixied. 

If the bath contains acetate of silver, free iodine liberates 
acetic acid in place of nitric acid ; and nitric acid added to 
such a bath neutralizes itself and displaces acetic acid. 

The actual quantity of acid liberated by collodion which has 
become brown from decomposition, is very inconsiderable, and 
it is quite a mistake to be continually neutralizing the nitrate 
bath. When, however, the trough which holds the bath is 
narrow, and the plates large, a minute addition of alkali may 
occasionally be required, to prevent the film from losing sen- 
sitiveness, and yielding weak metallic negatives. 

Alkaline condition of the Bath. — By " alkalinity" of the 
bath is meant a condition in which the blue tint is restored to 
reddened litmus-paper. This change, when rapid, indicates 



154 THEORY OF PHOTOGRAPHY. 

tliat a free oxide is present in solution, which by combining 
with the acid in the reddened paper neutralizes it and renaoves 
the red color. 

If a small portion of caustic potash or ammonia be added to 
a strong solution of nitrate of silver, it produces a brown pre- 
cipitate, which is oxide of silver. The solution, however, 
from which the precipitate has separated, is not left in a neu- 
tral state, but possesses a distinct alkaline reaction, since oxide 
of silver is sparingly soluble in water, and the solution re- 
stores the blue color of reddened litmus. Both oxide of sil- 
ver and carbonate of silver are also abundantly soluble in 
water containing nitrate of ammonia ; which salt is continually 
accumulating in the bath when compounds of ammonium are 
used for iodizing. 

An alkaline bath is fatal to success in photography, produc- 
ing that universal darkening of the film on applying the de- 
veloper to which the name of " fogging " has been given. 
Hence care must be used in adding to the bath substances 
which tend to make it alkaline. Collodion containing free 
ammonia, often sold in the shops, gradually does so. The use 
of potash or carbonate of soda to neutralize the bath, or even 
of chalk or marble, if salts of ammonia are present, has the 
same effect, when an excess is employed ; and hence a trace of 
acetic or nitric acid must afterwards be added. 

The mode of testing a bath for alkalinity is as follows : A 
strip of porous blue litmus paper is taken and held to the mouth 
of a bottle of glacial acetic acid until it becomes reddened ; it is 
then placed in the liquid to be examined, and left for ten 
minutes or a quarter of an hour. If free oxide of silver be 
present in solution, the original blue color of the paper will 
be gradually restored. This experiment must not be made in 
a strong light, or the litmus paper will darken, and tiie blue 
color be obscured. Indeed it is always somewhat difficult to 
examine a nitrate bath by test papers, since the pure nitrate of 
silver has a slightly alkaline reaction ; and hence the photo- 
graphic effect of alkalinity, viz., cloudiness of the image, will 
to the amateur afford a more certain guide. The writer be- 
lieves that the use of ammonia for the purpose of neutraliz- 
ing the bath is the most common cause of failure from alka- 
linity, few being aware that a single drop of strong ammonia 
will neutralize a comparatively large quantity of acid. 

Acetate of Silver in the Nitrate Bath. — In preparing a new 
bath, if the crystals of nitrate of silver are acid, it is usual to 
add an alkali in small quantity. This removes the nitric acid, 



THE NITRATE BATH. 155 

but leaves the solution faintly alkaline from oxide of silver. 
If acetic or nitric acid is then dropped in, it forms acetate or 
nitrate of silver by combination with the oxide. 

Acetate of silver is not formed by the simple addition of 
acetic acid to the bath, because its production under such cir- 
cumstance would imply the liberation of nitric acid ; but if 
an alkali be present to neutralize the nitric acid, then the 
double decomposition takes place, thus : — 

Acetate of Soda + Nitrate of Silver 
=Acetate of Silver + IsTitrate of Soda. 

Acetate of silver is a white, flaky salt, sparingly soluble in 
water. It dissolves in the bath only in small proportion, but 
yet sufficiently to aifect the photographic properties of the 
sensitive collodion film. The observance of the following 
simple rules will obviate its production in injurious quantity : 
First, when it is required to remove free nitric acid from a 
bath not containing acetic acid, a solution of potash or car- 
bonate of soda may be dropped in freely ; but the liquid must 
be filtered before adding any acetic acid, otherwise the brown 
deposit of oxide of silver will be taken up by the acetic acid, 
and the bath will be charged with acetate of silver. Secondly, 
in dealing with a bath containing both nitric and acetic acid, 
employ an alkali much diluted (liquor ammonise with ten parts 
of water), and add a single drop at a time, coating and trying 
a plate between each addition ; the nitric acid will neutralize 
itself before the acetic, and with care there will be no forma- 
tion of acetate of silver in quantity. 

The question is sometimes asked how acetic acid may be 
removed from the nitrate bath, and the nitric acid substituted. 
This operation is somewhat difficult to effect. Nitric acid, 
when present in excess, can be neutralized and converted into 
a nitrate, which is nearly or quite inert in photography ; but 
to neutralize acetic acid is to form an acetate, which is not 
inert, and the subsequent addition of nitric acid to such a 
solution again liberates acetic acid. Evaporation to dryness 
with a little nitric acid is the only means of effectually elim- 
inating the acetic acid from the bath. 

Organic Matter in the Nitrate Bath. — Nitrate of silver has 
an affinity for certain kinds of organic matter, and when such 
substances are present, the photographic action of the bath is 
in some way interfered with. 

Commercial crystallized nitrate of silver is frequently con- 
taminated with traces of an impurity, which is probably pro- 



156 THEORY OF PHOTOGRAPHY. 

diiced by organic matters falling into the nitric acid employed 
in dissolving the silver, liepeated vecrystallization is required 
to remove this substance. If allowed to remain it injures the 
sensitiveness of the film to feeble radiations, makes the nega- 
tive weak and metallic, reverses the action of the light, and 
produces either fogging or markings of various kinds, the 
result of irregular reduction of silver. 

Solutions of nitrate of silver often acquire organic contami- 
nation by being kept in troughs of gutta-percha. Pure gutta- 
percha seems to have little action upon nitrate of silver, but 
the commercial article is invariably impure. Caoutchouc 
seems also to be without action, but caoutchouc vulcanized 
with sulphur, such as is used for the tops of water-tight baths, 
will decompose the nitrate of silver by degrees. Baths injured 
by impure gutta-percha produce fogging of the film, streaks of 
irregular development, and quick discoloration of the solution 
of pyrogallic acid, attended with variations in the density of 
the negative picture. 

Albumen and gelatine soon decompose nitrate of silver, and 
hence the dipping of a few dry albuminized plates, or the 
floating of chloride papers intended for the printing process, 
upon a collodion bath, would effectually disorder it, and almdSt 
certainly give rise to fogging. 

Alcohol and ether react very slowly upon solution of nitrate 
of silver, and pyroxyline is almost without effect. Hence 
when pure collodion is employed, the bath may be kept for 
many months without much appreciable change. It should, 
however, be carefully excluded from light, or the sides of the 
bottle will be covered with a delicate layer of reduced silver. 
Methylated spirit of wine is seldom sufilciently free from 
volatile oil to remain long in the ba,tli without producing 
partial reduction. 

Collodion containing common resin has been recommended 
for use in the dry collodion process ; but the resin finds its 
way into the bath, and spoils it for the wet process, soon pro- 
ducing solarization of the most exposed parts of the plate, and 
altering the appearance of the film of iodide, so that it becomes 
pale and blue instead of being yellow and creamy. 

The effect produced by organic matters in the bath will be 
more intelligible if we explain that not only nitrate of silver, 
but also iodide of silver has an affinity for a certain class of 
these bodies. Hence when a collodion film is dipped in a bath 
of the kind described, a trace of organic matter is carried 
down and retained in the film. This applies especially to 



THE NITRATE BATH. 157 

albumen, which has so decided an affinity for iodide of silver 
that its presence will entirely alter the color of the film, 
and render it blue and transparent. A state of bath in which 
the collodion film, instead of being yellow and creamy, appears 
blue or opalescent, may be produced by organic matter, as well 
as by a deficiency of nitrate of silver. 

To remove organic matter from the bath, the following 
plan is often employed. The solution, having been rendered 
slightly alkaline by a solution of bicarbonate of soda or by 
ammonia, is exposed for two or three days to a bright sun- 
light in a transparent bottle, when the greater part of the 
organic compound separates in combination with the oxide of 
silver. This mode is generally successful in restoring a bromo- 
iodized collodion bath to a good working condition, but often 
fails in rendering it useful for a simply iodized collodion and 
pyrogallic developer. 

Use of Fumd Nitrate of Silver for the Bath. — Fusion has 
been resorted to with a view of expelling traces of nitric acid 
from the crystallized nitrate of silver, and thus lessening the 
trouble of preparing the bath. It must be observed, however, 
that decomposition of the nitrate is liable to occur in melting 
\i the crystals are not chemically pure. A trace of any organic 
substance, such as a bit of cork or a fragment of gelatine, ad- 
hering to the sides of the porcelain dish, would at the melting 
point of nitrate of silver remove oxygen and produce nitrite 
of silver; a safer plan is therefore to pulverize the nitrate and 
dry it in a hot-air bath, at about twenty degrees above the 
temperature of boiling water. Supposing the nitrate to be 
absolutely free from organic matter, there would be no danger 
of forming nitrite in the process of fusion, since the tempera- 
ture at which nitrate of silver decomposes is far above its 
melting point. 

Nitrate of silver which has been much decomposed in melt- 
ing, produces a great peculiarity of development. In the posi- 
tive glass process the whites are often solarized, and appear 
blue by reflected light; whilst in the negative process the 
image is highly intense, with overaction in parts like the sky, 
and the film usually fogs slightly towards the end of the de- 
velopment; on applying the developer the whole picture 
starts out instantly, and the solution of pyrogallic acid becomes 
muddy instead of assuming by degrees the color of sherry wine. 
These efliects are not unlike those previously described as due 
to certain kinds of organic matter in the bath, the reduction 
of silver by the developer being in both cases facilitated. 



CHAPTER YIII. 

ON POSITIVE AND NEGATIVE COLLODION PHOTOGRAPHS. 

"We use the terms " positive " and " negative " so frequently 
in photography to denote different kinds of chemicals and 
pictures, that it is important for the student to have a clear 
understanding of the meaning of these terms. 

A positive photograph may be defined to be a picture 
which gives a natural representation of the lights and shad- 
ows of an object as seen by the eye. 

A negative photograph, on the other hand, has the lights 
and shadows reversed, so that the appearance of the object is 
changed or negatived. 

The following diagrams will serve to make this obvious : 




Fig. 5. 



Fig. 6. 



Fig. 7. 



Fig. 5 is an opaque image drawn on a transparent ground ; 
Fig. 6 represents the effect produced by placing it in contact 
with a layer of sensitive chloride of silver and exposing to 
light, and Fig. 7 is the result of again copying this negative 
on chloride of silver. 

Fig. 7, therefore, is a positive copy of Fig. 5 obtained by 
means of a negative. By the first operation, the lights are 
reversed ; by the second, being again reversed, they are made 
to correspond with the original ; hence the possession of a 
negative enables us to obtain positive copies of the objects, 
indefinite in number and all precisely similar in appearance. 

The same photograph may often be made to show either as 
a positive or a negative. For instance, supposing a piece of 
silver leaf to be cut into the shape of a cross and pasted on a 



POSITIVE AND NEGATIVE COLLODION PHOTOGRAPHS. 159 

square of glass, the appearance presented by it would vary ac- 
cording to the way in which it is viewed. 

Fig. 8 represents it placed on a piece of black velvet ; Fig. 
9 as held up to the light and viewed by transmitted light. 

Positives, therefore, should be viewed by reflected, and 
negatives by transmitted light. 

All photographs, however, cannot be made to represent 
both positives and negatives. In order to possess this capabil- 
ity it is necessary that a part of the image should be transpa- 
rent and the other opaque, but with a bright surface. These 
conditions are fulfilled when an image on collodion is devel- 
oped by a reducing agent. 

Every collodion picture on glass is, to a certain extent, both 
negative and positive, and hence the processes for obtaining 
both varieties of photographs are in most respects the same. 





Fig. 8. 



Fig. 9. 



The conditions of success will be fully described in succeeding 
chapters. All that refers to obtaining positives on silver- 
chlorized paper will be treated of under the head of " Positive 
Printing." 

With these preliminary remarks, we are prepared to inves- 
tigate more closely the rationale of the processes for obtain- 
ing collodion positives and negatives. 



Section I. 
On Collodion Positives, or Ferrotypes. 

Collodion positives are sometimes termed direct, because 
obtained by a single operation. The chloride of silver acted 
upon by light alone is not adapted to yield direct positives, 
the reduced surface being dark and incapable of representing 
the lights of a picture. Hence a developing agent is neces- 
sarily employed, and the iodide of silver substituted for the 



160 THEORY OF PHOTOGEAPHT. 

chloride, as being a more sensitive preparation. Collodion 
positives are closely allied in their nature to Daguerreotypes. 
The difference between the two consists principally in the 
surface used to sustain the sensitive layer, and the nature of 
the substance by which the invisible image is developed. 

In a collodion positive the lights are formed by a bright 
surface of reduced silver, and the shadows by a biack back- 
ground showing through the transparent portions of the plate, 
when taken on glass, or by the natural dark color of the ferro- 
type plate. 

Two main points are to be attended to in the production of 
these photographs. 

First, to obtain an image distinct in every part, but of com- 
paratively small intensity. If the deposit of reduced metal be 
too thick, the dark background is not seen to a sufBcient ex- 
tent, and the picture in consequence is deficient in shadow. 

Secondly, to whiten the surface of the reduced metal as 
much as possible, in order to produce a sufficient contrast of 
light and shade. Iodide of silver developed in the usual way 
presents a dull yellow appearance, which is sombre and un- 
pleasing. 

The Collodion for Positives. — Ordinary collodion iodized 
for negatives and giving a thick yellow film, usually fails in 
taking good positives, if the exposure in the camera be suf- 
ficiently long to impress dark shadows, the lighter parts of the 
image develop with such rapidity that the gradation of tone is 
lost by excessive deposit of silver. The addition of nitric acid 
to the bath is to a certain extent a remedy, inasmuch as it les- 
sens the intensity of the reduction and gives the silver a spark- 
ling appearance ; nevertheless positives taken in this way are 
always unpleasing, and cannot be compared with a good Da- 
guerreotype in softness and delicacy. 

A better class of picture may often be obtained by diluting 
down a sample of collodion with ether and alcohol until it 
gives a pale-bluish film in the bath. The proportion of iodide 
of silver being in that case small, the action of the high lights " 
is less violent, and the shadows are allowed more time to im- 
press themselves. 

The employment of a very thin film for positives is not in- 
variably a successful process. The particles of the iodide of 
silver being closely in contact with the glass, unusual care is 
required in cleaning the plates in order to avoid stains ; and 
as the amount of free niti-ate of silver retained upon the sur- 
face of the film is small, circular patches of imperfect develop- 



COLLODION POSITIVES. 161 

ment, causing 1)lne and green stains, are liable to occur, un- 
less the reducing agent be scattered evenly and perfectly over 
the surface. Also if free iodine or organic substances which 
have a retarding effect on the action of light be present to a 
considerable extent, the collodion will not work well with a 
small proportion or iodide : in such a case a creamy and dense 
layer of the sensitive salt is required, to give a sufficient power 
of resistance. 

In attempting to dilute down the nitrate bath at the same 
time with the collodion, we gain in some respects an advantage. 
Excess of development is obviated, and the picture shows well 
on the surface of the film. The employment of a very weak 
nitrate bath (twenty grains of nitrate of silver to the ounce of 
water) in the positive process is not, however, advisable on the 
whole. It becomes necessary, on account of the small quan- 
tity of silver present, to exclude all free nitric acid, and even 
to avoid the employment of a collodion too highly tinted with 
iodine. On the other hand, with a strong nitrate bath, and a 
tolerably dense film of iodide of silver, a better result is 
often secured by the use of nitric acid, as will presently be 
shown. 

These and other like processes, in which films of iodide only- 
are employed, have now been almost superseded by others, 
founded on a peculiar property which bromide possesses when 
added in moderate proportion to an ordinary iodized collodion. 
It alters the molecular state of the silver reduced during de- 
velopment, rendering it metallic : the image also is superficial, 
and sufficiently translucent to give a perfect gradation of tone. 
When bromide is employed, in conjunction with sulphate of 
iron as a developer, the quality of the pyroxyline is not of the 
same importance as in the case of pictures taken with a simply 
iodized collodion. 

To produce a very faint and superficial image, a weak nitro- 
sulphuric acid must be used for the pyroxiline, but the objec- 
tion is that after a certain point of dilution with water, a qual- 
ity of pyroxyline is obtained which becomes opaque on drying. 
This is comparatively immaterial when transparent varnish is 
applied to the image, but it spoils the appearance of the picture 
when mounted without varnish, as is sometimes preferred. 

With regard to the quantity of bromide which may be in- 
troduced, the operator should be guided by the aspect of the 
developed image. If, when exposed long enough to form the 
shadows, the high lights appear too dense, more bromide must 
be added, whilst on the otlier hand, if the positive is gray and 



162 THEORY OF PHOTOGRAPHY. 

feeble, and this defect is not due to over-exposure, tlie propor 
tion of bromide may be reduced. The American operators, 
who are very successful with collodion positives, employ more 
bromide than is recommended in this work, but their pyroxy- 
line may possibly be made in a different manner. 

One effect produced by bromide in positive collodion, is 
seen in keeping the lines of the picture sharp and clear during 
development. When the bath is strongly acidified with nitric 
acid and a simply iodized collodion is employed, the image is 
often blurred by an irregular deposit of silver, giving an 
appearance like defective focusing. With collodion contain- 
ing mixed iodide and bromide in the proper proportions, this 
does not so frequently happen, the image remaining clear and 
free from stains. 

Does bromide increase or diminish the sensitiveness of the 
film to a weak light ? This is a question which can only be 
answered by considering the chemical state of the collodion, 
bath, and developer. With a weak solution of pyrogallic 
acid as a developer, and a dilute bath, bromide seems to 
diminish the sensitiveness considerably, but this appearance is 
delusive, since the latent image is really present, and simply 
requires a stronger reducing agent and more nitrate of silver 
to bring it out. On increasing the strength of the bath, there- 
fore, and using sulphate of iron as a developer, the whole of 
the details will show themselves after a minimum of exposure 
in the camera. 

There are states of collodion, also, in which the use of 
bromide may increase the sensitiveness, viz., when decomposi- 
tion has taken place. Every practical operator learns by 
experience that positive collodion containing bromide does 
not lose sensitiveness alter iodizing in the same rapid manner 
as negative collodion prepared without bromide. Doubtless 
the difference in the developing solutions — sulphate of iron in 
the one case and pyrogallic acid in the other — has something 
to do with this result ; but a peculiar action of the bromide in 
rendering the collodion independent of organic changes due 
to keeping after iodizing must be allowed. 

A comparatively newly-iodized collodion is seldom in the 
best possible state for taking positives ; for, if it be colorless, 
or nearly so, the plates will show clouding after development. 
By keeping the collodion for several weeks, a portion of the 
iodine is liberated, which has a great effect in giving brilliancy 
to the picture, by preserving the shadows from the reducing 
action of the developer. 



COLLODION POSITIVES. 163 

The Nitrate Bath. — An ordinary neutral 30-grain solution 
of nitrate of silver, such as is used for negatives, may be 
employed also for positives, and especially so if the collodion 
be purposely kept after iodizing until it assumes a yellow tint, 
and liberates enough iodine to prevent the clouding which 
might otherwise take place in the absence of acid. Negative 
baths, however, which contain acetate of silver, or any kind 
of organic matter, or which have been made from nitrate of 
silver previously decomposed by strong fusion, are not well 
adapted for positives, since these conditions promote solariza- 
tion, a defect which shows itself in positives by discolored 
whites on the most exposed parts of the film. 

When the nitrate bath is made purposely for direct posi- 
tives, it is always better to acidify it with nitric acid rather 
than with acetic acid, as used for negatives. The nitric acid 
diminishes the density of the deposited image, renders it 
whiter and more metallic, and asissts in preserving the clear- 
ness of the plate. The mistake has sometimes been made of 
using too much acid in proportion to the strength of the bath, 
which not only lessens the sensitiveness of the film, but also, 
as before shown, promotes a blurring of the image, and a fog- 
ging from irregular reduction. Even with a collodion con- 
taining bromide, there will be a tendency to staining and 
imperfect development, if the amount of nitric acid be carried 
as far as is sometimes recommended, in the case of a bath of 
30 grains to the ounce. 

Many successful operators use a bath of 40 grains to the 
ounce for collodion positives, in order to shorten the exposure 
in the camera and give brilliancy to the image. This solution 
will bear a far larger addition of nitric acid than the ordinary 
bath of 30 grains ; and, always supposing bromide to be 
present in the collodion, satisfactory pictures may be obtained 
with a quantity of nitric acid as large as two minims to the 
ounce. Neither is the sensitiveness very much impaired by 
the excess of acid, thus showing how many conditions have to 
be taken into account in considering the formation and devel- 
opment of the collodion picture. With a bath very strongly 
acidified, it would be impossible to get any result if the pro- 
portion of nitrate of silver and iodide of silver in the col- 
lodion film were both reduced to one-half. The acid and ni- 
trate of silver must, in fact, be balanced against each other, 
and when the collodion film is lessened in opacity, the devel- 
oper must be considerably strengthened, otherwise the latent 
image, although actually present, cannot be made to appear in 
the half-shadows. 



161 THEORY OF PHOTO&RAPHY. 

Our experience is not favorable to the employment of a 
40-grain batli in the positive glass process as far as begin- 
ners in the art are concerned. We find that there are fewer 
failures when the bath contains 30 grains of nitrate of sil- 
ver to the ounce, and is only slightly acid. Stains of reduced 
silver at the edges of the plate are not easily avoided in the 
case of a strong and acid bath, and if the developer is too 
weak, there are metallic spangles on the shadows. 

Developers for Collodion Positives. — In the case of a col- 
lodion which does not contain bromide, pyrogallic acid is oc-' 
casionally employed as a developer. When mixed with acetic 
acid, as is usual for negative pictures, it produces a surface 
which is dull and yellow ; but this may be obviated by substi- 
tuting a small quantity of nitric acid for the acetic (pyrogallic 
acid, 2 grains; nitric acid, 1 drop; water, 1 ounce). The 
surface produced by pyrogallic acid with nitric acid is lustre- 
less, bnt very white, if the solution be used of the proper 
strength. On attempting to increase the amount of nitric 
acid, the deposit becomes metallic and the half-tones of the 
picture are injured, since pyrogallic acid, although an active 
developer, does not allow of the addition of mineral acid to 
the same extent as the salts of iron ; it requires also, when 
combined with nitric acid, a fair proportion of nitrate of sil- 
ver on the film, or the development will be imperfect in parts 
of the plate, producing green and blue stains; if such stains 
occur, a few drops of a solution of nitrate of silver may be 
added to the developer before use. 

iSuljphate of Iron, — This salt is a more energetic reducing 
agent than the last, and is better fitted for employment with 
a collodion containing a portion of bromide, the action of 
bromide being to retard the development. To produce, by 
means of sulphate of iron, a dead white tint, with absence of 
metallic lustre, it may be used in conjunction with acetic acid. 

The addition of nitric acid to sulphate of iron modifies the 
development, making it more slow and gradual, and producing 
a bright, sparkling surface of reduced silver. Too much of 
this acid, however, must not be used, or the action will be 
irregular. The nitrate bath also should be tolerably concen- 
trated, in order to compensate for the retarding effect of nitric 
acid upon the development. The blue and transparent films 
of iodide of silver are not well adapted for positives to be de- 
veloped in this way. Such films require the most vigorous 
developer possible : acetic acid should therefore be used in- 
stead of nitric acid. 



COLLODION POSITIVES. 165 

Protonitrate of I?' on. — This salt is remarkable as giving a 
surface of brilliant metallic lustre without any addition of free 
acid. Theoretically, its action may be considered as closely 
corresponding to that of sulphate of iron with nitric acid 
added. There are, however, slight differences between them, 
w^hich are in favor of the protonitrate, as regards the color of 
the image. 

The nitrate is, of all the protosalts of iron, by far the most 
feeble developer, and is se.dom used alone in photography. 
In the case of bromo-iodized collodion, very little dependence 
must be placed upon the protonitrate of iron as far as reducing 
metallic silver is concerned, but it may be added to solution of 
sulphate of iron when the peculiar metallic tone which it im- 
parts is desired. Beginners often fail in using this developer, 
from not allowing a sufficient excess of the sulphate of iron in 
its preparation, and from supposing that the protonitrate is 
equally as strong a reducing agent as the protosulphate 
whereas it is at least twenty times weaker. 

The Tone and Color of Positives. — The same chemicals 
yield such different results in the hands of various operators, 
that some have spoken of manipulation and practice as more 
essential than theory. Doubtless the exact time of exposure 
in the camera is important, because a short exposure always 
produces an image of a whiter color and a more transparent 
shadow than a long exposure. In the case of negatives, the 
image, viewed by transmitted light, is often of a jet black 
when under-exposed, but of a ruby red when over-exposed. So 
in positives, it is always brighter and more metallic when the 
action of the light is stopped at the proper time. There are 
other causes of variation in the color of the positive image, 
such as changes in the actinic intensity of the light, and the 
operator may expect to produce a better quality of picture 
when the light is strong. 

The gradation of tone in collodion positives will also be 
affected by the mode in which the light falls upon the subject ; 
for when it possesses great actinic power, there is always a 
tendency to an excessive reflection from the high lights, and 
if this be not counteracted by an arrangement of curtains, to be 
described in another part of this work, it will require the 
utmost attention to the state of the chemicals to prevent 
excess of intensity and loss of good shading. 

The mode of developing the positive image has an eifect 
upon the gradation, independently of the exposure or of pecu- 
liarities in the collodion. The theory is as follows : When 



166 THEOKT OF PHOTOGRAPH Y. 

tSe nitrate of silver is present upon the film in large quantity 
relatively to the sulphate of iron, the tendency is to give great 
contrast of image, and to produce a bold picture, which ap- 
pears to stand out from the glass. The deposit of silver in 
such a case falls more abundantly upon the high lights, and 
less so upon the shadovt's, so that the shading in the face and 
lighter parts is ofteu lost. On the other hand, when the 
quantity of nitrate of silver is much reduced in relation to the 
sulphate of iron, the image develops more slowly, and is soft 
and full of half-tone ; in extreme cases the high lights are not 
sufficiently opaque, and the features of the sitter, after backing 
up with the black varnish, are those of a negro. This rule 
applies to every kind of developed picture, and the practical 
inference from it is, tliat when the positive is too vigorous, 
and the details in the face are lost, a large quantity of develop- 
ing solution should be scattered over the film so as to wash off 
a portion of the free nitrate of silver. Experimental trial 
will convince the operator how extremely minute is the pro- 
portion of nitrate of silver actually required for the formation 
of the image ; but no exact directions can be given, since the 
quantity will vary with the density of the sensitive film, the 
brightness of the light, the amount of retarding acid, and the 
strength of the developer. 

Fixing Agents for Positives. — Hyposulphite of soda is not 
usually employed for fixing collodion positives, since it con- 
tains sulphur and is somewhat unstable ; hence the image is 
liable to be superficially darkened to an appreciable extent. 
Cyanide of potassium is free from this objection, and always 
produces a whiter picture than hyposulphite. The strong 
solvent powers of cyanide of potassium must be borne in mind 
when using it as a fixing agent ; for if the solution be too con- 
centrated or allowed to remain a long time upon the plate, the 
middle tints first become whitened from conversion into cyanide 
of silver, and immediately afterwards begins to dissolve, 
i^either should the plate be exposed to a strong light whilst 
the cyanide is upon it, as the action of light favors the conver- 
sion of the image into cyanide of silver. Sulphocyanides of 
potassium and ammonium will probably be found useful fixing 
agents for positives, as they are far less likely than the cyanide 
to endanger the delicate half-tones of the picture. 

A Process for Whitening Positives hy Corrosive Subli- 
mate. — In place of brightening the positive ima^e by modi- 
fying the developer, it was proposed some time since by Mr. 



COLLODION NEGATIVES. 167 

Arclier to effect the same object by the use of the chloride of 
mercury, HgCL 

The image is first developed in the usual way, fixed, and 
washed. It is then treated with a solution of the chloride 
(thirty grains dissolved in an ounce of hot water), the effect of 
which is to produce almost immediately an interesting series 
of changes in color. The surface first darkens considerably, 
until it becomes of an ash-gray, approaching to black ; shortly 
it begins to get lighter, and assumes a pure white tint, or a 
white slightly inclining to blue. 

The rationale of this reaction appears to be, that the chlorine 
of the mercurial salt divides itself between the mercury and 
the silver, producing a compound of calomel and chloride of 
silver, which is not affected by light. 

Positive photographs whitened by chloride of mercury have 
usually more or less of a bluish tone, but this may be over- 
come by associating other chemicals with the corrosive subli- 
mate. A more serious objection is the instability of the pic- 
ture after whitening, since it has been stated that these images 
are liable to fade unless special precaution be taken. 

Section II. 

ON COLLODION NEGATIVES. 

As in the case of direct positives, we require an image which 
is feeble though distinct, so, on the other hand, for a negative, 
it is necessary to obtain one of considerable intensity. In the 
chapter immediately following the present, it will be shown 
that in using glass negatives to produce positive copies upon 
chloride of silver paper, a good result cannot be secured unless 
the negative is sufficiently dark to obstruct light strongly. 

There are various conditions affecting the formation and de- 
velopment of a photographic negative, which ought to be stu- 
died by one who aims at perfection in the art. Some of these 
are : the quality of the light at the time of taking the picture, 
the focal length and aperture of the lens, the state of the at- 
mosphere, etc. To meet the different cases which may arise, 
the collodion, the nitrate bath, and the developing solution 
may each or all be considerably modified. 

With regard to the first of these conditions we may observe 
that the mode in which a negative develops, and its color when 
the process is completed, vary much with the actinic intensity 
of the light. Pictures taken by a short exposure in a strong 



168 THEORY OF PHOTOGRAPHY. 

light, develop easily under tlie pyrogallic acid. The first de- 
posit of silver is often of a red color when held against the 
light, and this influences the subsequent precipitation, so that 
the negative continues to darken until the whole of the nitrate 
of silver in the film has been decomposed. When examined . 
after fixing, it sliows a plum-color or yellow bloom by reflected 
light, and is often brown by transmitted light. In dull wintry 
weather, on the contrary, and especially when the atmosphere 
is loaded with aqueous vapor, the negative picture is slow in 
appearing under the action of the developer, and the image 
first formed is rather of a violet-blue than of a red color. In 
consequence of this, the subsequent deposit of silver is like- 
wise of a violet-blue, and a much longer time is expended in 
bringing the development up to the proper point. The par- 
ticles of precipitated silver are always larger when the reduc- 
ing process is slow, and thus negatives taken in a bad light often 
prove in printing to be less intense than they appear, whereas 
those taken in a bright light are usually more intense chemi- 
cally than visually. 

When the strength of the light falls very low indeed, as, for 
instance, in working in a room covered in with glass of a 
greenish tint, fatal to quick photographic action, the darker 
parts of the picture fail to impress the film, and the negative 
shows only patches of black and white ; or, with a more sensi- 
tive collodion, the whole picture appears, but is extremely 
feeble and indistinct, a deposit of silver falling upon the 
shadows, and giving a cloudy aspect. On the other hand, in a 
light of extreme brilliancy, reddening from solarization is to 
be apprehended, the actinic rays being reflected from the high 
lights of the subject in a greater proportion than from the 
shadows ; this may happen especially with a sky free from 
fleecy clouds, which, when present, have a great effect in il- 
luminating the shadows, and diminishing excessive contrast. 
When solarization of negatives occur from great intensity of 
light, the image often starts out almost instantaneously on ap- 
plying the pyrogallic acid, and after fixing by the hyposulphite 
of soda, exhibits an intensely red appearance of the high lights, 
with a steel-blue color of the same parts viewed by looking 
down upon them. 

We notice in the second place the focal length and aperture 
of the lens, as a condition infiuencing the quality of the de- 
veloped negative. If, for instance, two cameras be pitched 
side by side, at the same view, one being a stereoscopic in- 
strument with a single lens of 4|^-inch focus, and the other fit- 



COLLODION NEGATIVES. 169 

ted with a twenty-incli lens, suitable for glasses of twelve inches 
diameter, the plates, even when properly exposed, will behave 
differently under the action of the developer, the stereoscopic 
picture showing by far the greater intensity, contrast of tint, 
and tendency to solarization. The relative power of the lenses 
makes the difference, the short focus lens producing a brilliant 
image which impresses the sensitive film in less than a third 
of the time required by the other. A second experiment, 
equally instructive, may be made by surrounding a group of 
statuary with a dark drapeiy, so as to produce a strong contrast 
of light and shade. On copying such an object with a full 
aperture of a short -focus lens, it will be found almost impos- 
sible, unless the composition of the collodion be modified in a 
manner presently to be explained, to get the whole of the pic- 
ture simultaneously. Either the whites will be well rendered, 
and the blacks wanting, or by a longer exposure the shadows 
will be well brought out, but the lights solarized. JNow take 
the same lens and cut down the effective aperture by a middle- 
sized diaphragm, allowing proportionally longer time in the 
camera, when a perfect negative will be produced. This rule, 
then, may be stated as follows : — A camera image of great 
actinic intensity often produces a collodion negative with 
exaggerated contrast of light and shade, and conversely an 
image of low intensity tends to give a picture which is too 
uniform, and deficient in extreme tints. Availing himself of 
this knowledge, the skillful operator alters the size of his dia- 
phragm according to the quality of the picture desired, and 
remedies solarization by diminishing the supply of light. 

The amateur, on commencing photography, is often at a loss 
to understand apparent discrepancies in the formula emploved 
by various operators, and in their general mode of working. 
In one instance a plate of twenty inches by seventeen is de- 
veloped to the full intensity with a single application of the 
reducing agent ; in another, a negative of half that size is 
found to require repeated treatment with pyrogallic acid and 
nitrate of silver, and to occupy ten or fifteen minutes in reach- 
ing the full opacity. Supposing the lens and chemicals to be 
the same in each case, the difference may depend upon the 
character of the subject. When a distant landscape is photo- 
graphed, a large number of rays of light are concentrated upon 
the film ; but if an object like an old picture, or a faded draw- 
ing, be copied without any reduction in size, the light is very 
feeble. No careful timing of the exposure will cause two such 
negatives to develop in the same way, because the long-con- 



170 THEOEY OF PHOTOGRAPHY. 

tinued action of a weak light in the camera does not corres- 
pond to the shorter action of an intense light. The molecules 
of iodide of silver are differently affected in the two cases, and 
consequently they behave differently when treated with the 
mixture which constitutes the developer. 

The Collodion for Negatives. — A perusal of the last chap- 
ter will show that the properties of negative collodion vary 
much with the mode of preparing the pyroxyline. Dis- 
tinguishing terms, however, are always useful, and hence we 
propose to speak of pyroxyline with, and pyroxyline without, 
organic reaction towards the salt of silver ; the former being 
the variety which yields the intense negative images. 

Pyroxyline without organic reactions may be expected to 
give the greater sensitiveness of the two, and the better keep- 
ing qualities after iodizing. It is, however, difficult to use it, 
in consequence of the increased liability to fogging, and spots 
of all kinds. Also, when the intensity and contrast of the 
image are too small, as frequently happens with such collo- 
dion, they cannot easily be increased, whereas excessive in- 
tensity may readily be reduced. The pyroxyline with organic 
reactions is, therefore, preferred by the majority of operators. 

For the sake of greater simplicity, the author has given only 
one formula for negative pyroxyline, in Chapter XL, of this 
work. This formula appears to answer well for normal collo- 
dion, applicable to all purposes. Those, however, who wish 
to prepare a pyroxyline suitable for any one especial kind of 
work, will find the subject examined more fully in another 
part of this volume. 

Independently of the pyroxyline, negative collodion may be 
described as of two kinds, the simply iodized, and the bromo- 
iodized collodion. Each has its advantages and likewise its 
defects. The first gives the greater sensitiveness, and also the 
greater intensity, but it is more liable to spots and markings, 
and the purity of the chemicals is of greater importance than 
when bromo-iodized collodion is employed. 

Let us examine the above points more minutely. It must 
be allowed that the addition of a bromide to negative collodion 
impairs the sensitiveness. Much, indeed, depends upon the 
mode of developing, and even when the reducing agent is 
strong, and the nitrate of silver abundant, the bromo-iodized 
collodion is still inferior in that respect. If such a collodion be 
found on trial to yield a picture in eight seconds, a pure, 
simply iodized cadmium collodion will probably give the same 
picture in five seconds. Hence, in the dull winter months, 



COLLODION NEGATIVES. 171 

many are necessitated to employ iodides alone, who at more 
favorable times use combinations of iodide and bromide. 

The excessive sensitiveness of simply iodized collodion, how- 
ever, is not seen in perfection unless all the materials are pure, 
and hence some have stated that the addition of a small portion 
• of a bromide increases the sensitiveness. Experiments leading 
to such a conclusion were probably made with a collodion 
which from organic decomposition of the pyroxyline or other 
causes, had acquired the property of producing an intense nega- 
tive, and in this case the action of the bromide, as already 
shown in previous pages, is peculiar. A pyroxyline without 
strong organic reactions is proper for the most sensitive iodized 
collodion, and the ether must also be very pure. When this 
extreme purity of the ether cannot be secured, iodide of cad- 
mium should be used in place of iodide of potassium, to pre- 
serve the fluid in a neutral and colorless condition. The manu- 
facturer of collodion who examines its properties shortly after 
its preparation, finds little or no diiference between the iodide 
of potassium and the iodide of cadmium. The purchaser, 
however, usually esteems the latter to be superior as regards 
sensitiveness, because the plain collodion has often been kept 
for a time before he obtains it, and has acquired the property 
of displacing iodine ; hence with the potassium iodizer the 
color changes quickly to orange-yellow, and the action of the 
light is retarded. Iodide of cadmium, in consequence of its 
stability, would have superseded the other iodides, had it not 
been for its glutinizing action on pyroxyline — a serious objection 
in covering large surfaces of glass. 

The effect of bromide in diminishing the intensity of the 
image has already been considered in the section on positives. 
It is often used for the same purpose in negative collodion, be- 
cause when the light is strong and the lens powerful, the in- 
tensity and contrast may be in excess, and if so, either a black 
and white picture without middle tints, or a red picture de- 
fective from solarization will be obtained. The bromide 
effectually remedies both these causes of failure ; frequently, 
indeed, it originates an error in the opposite direction. On 
looking at a negative taken with a bromo-iodized collodion, 
we see that the peculiar metallic effect given by the bromide 
is most evident in the half -shadows of the picture, where the 
light acts feebly. Provided there be a sufficiency of light, 
enough intensity will be obtainable in presence of bromide, 
but when the light is too much reduced, the image will be 
rendered weak and translucent thereby. A negative must pos- 



172 THEOKT OF PHOTOGKAPHY. 

sess a certain amount of intensity, and also a proper color, if it 
is to possess what the photographer terms " good printing 
qualities," These qualities cannot be secured by merely 
pushing the development, and piling up particles of metallic 
silver; much will depend upon the color of the image in its 
early stage, which should be of a soft red, and should appear* 
nearly homogeneous when magnified. The employment of 
bromide should be suspended when the particles of silver be- 
come large and crystalline, since this condition of image is too 
permeable to the chemical light in the process of printing. 

The use of bromides in negative collodion must be regulated 
by the nature of the pyroxyline, as well as by the brilliancy of 
the light. The more marked the organic reactions of the 
pyroxyline, the greater the ]3roportion of bromide admissible. 
This rule is the same as that laid down for collodion positives, 
and the directions are to examine the negative, and if the con- 
trast between the high lights and the shadows is too marked, 
to increase the quantity of bromide. There are varieties 
of pyroxyline which will not bear any addition of bromide, 
the contrast of the image being too small, even with simple 
iodide. 

Apart from all considerations of density of negative depend- 
ing either upon the light or the pyroxyline, it must be ad- 
mitted that the employment of bromide in negative collodion 
is most desirable for other and distinct reasons. The latent 
image produced in the camera appears to be of a more definite 
description upon a bromo-iodized than upon a simply iodized 
collodion. In the case of the latter, disturbances ot all kinds 
are apt to arise during development ; the deposit is defective 
in one part of the plate, and irregular in another, so that to 
secure a negative free from all blemish is a rare occurrence. 
Much, indeed, depends upon the skill of the operator, and the 
purity of his chemicals, but this does not lessen the value of 
the bromide, which undoubtedly exercises a remarkable in- 
fluence in preventing, not only over-action of light, but like- 
wise transparent markings, vertical lines, and spots of all 
kinds. Iodide of silver, when associated with bromide, re- 
ceives the molecular modification in the camera somewhat 
tardily, but when once impressed, it retains the image with 
greater force, and is not equally liable to the reception of 
false images from moisture, or traces of organic matter upon 
the glass. 

Although in anticipation of our subject, we may here re- 
mark, in addition, that those minor decompositions and impur- 



COLLODION NEGATIVES. 173 

ities in the negative nitrate bath, which so frequently disturb 
the image in the case of a simply iodized collodion, will not 
produce the same effect when the lilm contains iodide and bro- 
mide conjoined. To prove the truth of this assertion, the 
Mn-iter on one occasion collected several impure nitrate baths, 
none of which would yield a j)erfect negative witli ordinary 
collodion, and yet he succeeded in every case in taking a good 
impression W'ith bromo-iodized collodion. Bromide, therefore, 
may he viewed as a useful adjunct when retarding impurities 
of various kinds are present, against which the unassisted 
iodide would be unable to contend. 

The foregoing remarks on the comparative advantages of 
iodized and bromo-iodized collodions apply more particularly 
to their use with a pyrogallic developer. Experience has 
proved that tlie former, except under the exceptional condi- 
tions mentioned, is better adapted for a pyrogallic, and the 
latter for an iron developer. In the practical instructions it 
has, therefore, been deemed advisable to give separately the 
formulae and the mode of manipulation which have been found 
most applicable to each kind. 

Changes in Negative Collodion after Iodising. — All col- 
lodion loses sensitiveness more or less after iodizing, and es- 
pecially so when the pyroxyline is unstable and liable to part 
with a ]3ortion of its peroxide of nitrogen ; also when the ether 
is impure and in an ozonized condition. Practically we esteem 
a collodion iodized with iodide of cadmium as uniform in prop- 
erties, since if the chemicals are of good quality it will retain 
its sensitiveness nearly unchanged for twelve months after 
iodizing. Next in stability to the cadmium collodion stands 
the bromo-iodized collodion, which remains unchanged for a 
far longer time than a simply iodized collodion, and will retain 
a fair share of sensitiveness tor many weeks. 

Negative collodion produces as a rule a more intense image 
when kept for a time in the iodized state ; and this is true not 
only of collodion iodized with the alkaline iodides, but also 
of that containing iodide of cadmium. . Bromo-iodized collo- 
dion also gains in intensity as it gradually decomposes, and tlie 
quality of the negatives is usually better after a few weeks' 
keeping than at first ; they have more of a red or black tone, 
and exhibit greater contrast. 

When alkaline iodides are used, both the simple and the 
bromo-iodized collodion become more limpid by keeping. 
This is an advantage in coating full-sized plates, and since the 
setting powers of the collodion are lessened, it becomes far 



1Y4 THEOBT OF PHOTO&KAPHY. 

easier to cover a large surface Lefore gelatinization ensues. 
Old iodized collodion of this kind is likewise more porous and 
permeable by the developer than newly iodized, which affords 
another reason why it is especially suitable for large plates. 

Clearness of the transparent part in negatives, with in- 
creased sharpness of outline, are both eifects of keeping collo- 
dion after iodizing. These peculiarities, as well as the com- 
parative absence of spots and markings, are doe partly to the 
" organic reactions" developed in old collodion, and partly to 
the acid state of the film, w^hen the collodion is brown from 
free iodine. 

On keeping simply iodized negative collodion for a much 
longer time, the amount of free iodine often becomes so great 
that the color deepens to a red, and a portion of nitric acid is 
liberated on dipping the film into the bath, sufficiently large 
to lower the density of the negative, and make it grey and 
metallic. 

The Ifegative Nitrate Bath for Iodized Collodion. — In a 
previous chapter the chemistry of the nitrate bath was ex- 
plained with the action of organic substances upon it, and the 
marked effect which they produce upon the development of 
the negative image. 

Supposing the nitrate of silver to be chemically pure, a 
question arises as to the proper state of the negative bath as 
regards strength, acidity, presence of acetate of silver, etc. 
On these points opinions are divided. Much depends upon 
the quality of the collodion, and therefore the observations 
now to be made may be said to apply principally to the use of 
a simply iodized collodion prepared by the formulse given in 
this work, and not to a collodion containing both bromide and 
iodide. 

Negative baths have been used with as much as forty grains 
of nitrate of silver to the ounce of water ; thirty grains, how- 
ever, is quite sufficient, and any proportion beyond this would 
only add to the rapidity of development and tendency to 
staining in hot weather. When a bath, originally made with 
thirty grains of nitrate of silver to the ounce of water, has 
been much used, the proportion of nitrate will be found on 
analysis to have been considerably reduced. Ordinary collo- 
dion dipped in such a bath produces a less creamy film than 
usual, and the sensitiveness will be found to be much dimin- 
ished. 

The reaction of the bath to test-paper ought to be either 
neutral or slightly acid, an alkaline reaction always being in- 



COLLODION NEGATIVES. 175 

jnrious. A neutral batli produces a more dense negative in 
dull weather, but is apt to give solarization in bright sunsliine. 
The presence of acetic acid obviates this in great measure. 
An impressiun is not uncommonly entertained that any acid 
in the bath greatly diminishes the sensitiveness. With the 
collodion described subsequently, the writer does not iind such 
to be the case, and therefore, in hot weather, and with a neu- 
tral colorless collodion, he recommends that acetic acid should 
be used in the bath to prevent rapid discoloration on applying 
the developer. A minim of the glacial acid may be dropped 
into each four or five ounces, if the subject be well lighted. 
In using an acid bath, however, the developer should be pro- 
portionally strengthened if the temperature or the light after- 
wards fall. 

The use of nitric acid in the negative bath has been usually 
condemned as interfering with the precipitation of the silver 
in the opaque form, but in this instance also everything de- 
pends upon the collodion and the light. Pyroxyline with 
organic reactions tends so strongly to produce a brown or red 
image in bright sunshine, that the collodion can be worked 
successfully in a bath containing a trace of nitric acid. In 
circumstances favorable to rapid development and solarization, 
such as stereoscopic photography with lenses of a short focus, 
the presence of a minute quantity of nitric acid is often a posi- 
tive advantage, and no marked effect in preventing the de- 
velopment of details in the shadows will be perceived. 

The employment of acetate of silver in the bath met with 
much favor in the early days of photogi-aphy, but principally 
so because the preparation of pure nitrate of silver and the 
chemistry of collodion were not understood. There is, in fact, 
an impurity common in commercial nitrate of silver which 
lowers the density of negatives, and since acetate always in- 
creases the density, its use has been found in such cases to, be 
an improvement. So, again, pyroxyline made at low tempera- 
tures and in weak acids produces a violet image, and hence 
acetate of silver, which changes the color from blue to red, in- 
creases the intensity. When we add to this the fact that the 
presence of acetate frees the bath effectually from nitric acid, 
and thus enables a collodion to be used which has been kept a 
long time, and is very brown from excess of free iodine, it is 
not difficult to understand why it has been so extensively em- 
ployed. If pure recrystallized nitrate of silver be selected in 
the first instance, and proper attention paid to the state of the 
collodion, both as regards the length of time it has been kept 



176 THEORY OF PHOTOGRAPHY. 

after iodizing, and the quality of the pyroxyline, full intensity 
of negative may be obtained without the use of acetate, even 
under somevs^iiat unfavorable conditions. 

The objections to the employment of acetate of silver in the 
bath are that it renders the solution more liable to change by 
keeping, favors red solarization in sunny weather, and at high 
temperatures increases the chance of spots, markings and dis- 
coloration of the developer on touching the film. 

Tlte Negative Nitrate Bath for Bromo-iodized Collodion. — 
Experience has shown that a nitrate bath specially prepared 
for an iodized collodion with pyrogallic developer, is not the 
best suited for a bromo-iodized collodion with iron developer, 
and vice versa. In the former case, the aim is to obtain condi- 
tions of the nitrate bath which will combine the greatest sensi- 
tiveness with sufficient intensity of negative at one development. 
When the chemicals are pure, such a combination is easily at- 
tainable. In the latter case, density and sensitiveness com- 
bined seem incompatible. But as intensity of image is quite 
under control, by a subsequent redevelopment, it is considered 
more important to prepare the nitrate bath almost entirely with 
the view of securing the highest sensitiveness. Acetate of 
silver and acetic acid, which in the nitrate bath for iodized 
collodion often exercise a beneficial effect in increasing both 
density and sensitiveness, here act differently. They increase 
the intensity of image, but tend to lengthen the exposure. A 
small proportion of nitric acid, on the other hand, materially 
increases the sensitiveness, and, at the same time, lowers the 
vigor of the image ; but since the latter can be raised to any 
extent by subsequent development, it is generally sacrificed in 
the first instance in order to secure the former. Directions 
for the preparation of this bath will be f>mnd in another 
page. 

Bevelojnng Solutions for Collodion Negatives. — Three 
formulae are given subsequently, each of which has its ad- 
vantages. The first contains pyrogallic acid with acetic acid ; 
the second, pyrogallic acid with citric acid ; and the third, sul- 
phate of iron with acetic acid. 

Developers for Iodized Collodion. — Pyrogallic acid with 
acetic acid is the form most commonly employed, the use of 
the acetic acid being to moderate the violence of the action, 
and to preserve those parts of the iodide which have not been 
touched by light. It has also the advantage of causing the 
solution to flow easily upon the film, thus forming a substi- 



NEGATIVE DEVELOPERS. ITT 

tiite for alcohol, which would otherwise be required to pre- 
vent greasiness and streaks. The ordinary strength of the 
solution of pyrogallic acid is 1 grain to the ounce of water; 
but as regards the quantity of acetic acid, the practice of 
operators varies. The intensity is greatest when the mini- 
mum proportion of about 5 minims to the ounce is used ; but, 
in this case, a portion of spirit will be required in covering 
large plates. With a negative collodion giving abundance of 
intensity, it is better in every respect to employ the full 
strength of acid, viz., from 20 to 30 minims to the ounce, by 
which the reduction will be rendered more even, and stains of 
irregular action prevented. 

Attention should here be drawn to the decomposition which 
commercial pyrogallic acid experiences by keeping. In the 
course of a very few days in liot weather the solution becomes 
yellow, and not only loses, in some measure, its power of 
developing the weakest radiations, but rapidly discolors the 
nitrate upon the film. Solution of sulphate of iron also 
becomes yellow by kee23ing ; but, in this case, no injurious 
effect results, except in the weakening of the developer. 

Pyrogallic acid with citric acid may be viewed as a weaker 
reducing agent than the last, and one less likely to develop 
the dark shadows after a short exposure. Citric acid, in fact, 
is an agent of far greater power in retarding reduction of 
silver than acetic acid, and three-quarters of a grain wdll be 
found more than equivalent to 20 minims of the latter. This 
property of citric acid is an advantage when working at high 
temperatures, as, for instance, in a tent heated by the direct 
rays of the sun. The ordinary developer then acts so quickly 
that it is impossible to cover the plate before the reduction 
begins, and the discoloration on touching the film is rapid, so 
that the pictures are almost invariably weak and red, wdth 
stains and spots. In this state of things, a stronger acid wall 
be found serviceable ; and, although the ordinary proportions 
are twice as much of pyrogallic acid as of citric acid, yet in 
extreme cases the relative amount of acid may be doubled, 
and a grain of each constituent of the formula be dissolved 
in an ounce of water. The solution will fiow evenly over the 
film on adding alcohol, and the image will not appear until 
after an interval of 20 or 30 seconds. At so high a tempera- 
ture, the reducing power of even a feeble developer will be 
quite enough to bring out the shadows. 

Citric acid in the developing solution changes the color of 
the negatives from brown to blue, and in consequence the real 



178 THEORY OF PH(3TOGRAPHY. 

intensity of the image is somewhat less than it appears. It 
also has a marked effect in preventing red solarization in a 
brilliant light, and in preserving the surface of the him from 
fogging. Ilence it is particularly adapted for distant land- 
scape views, including sky and water, or for other subjects well 
lighted ; whilst, on the other hand, it is not suited for working 
in a glass house in a bad light and in cold weather, nor for 
copying works of art with long-focus lenses, nor for taking in- 
teriors. In all such cases we may anticipate, when using a de- 
veloper containing citric acid, that the image in its early stage 
of development will be weak and metallic, showing nothing of 
that tone of red which is so essential to the proper continu- 
ance of the precipitation. There will also be a loss of half- 
tint, from the reducing agent being too weak to bring out the 
darker portions of the image. 

Pyrogallic acid with citric acid forms a good developer for 
cadmium landscape collodion. Free iodine being absent from 
this collodion, the film on leaving the bath is nearly neutral, 
and no nitric acid is present. Consequently the high lights 
over-act, and render the image very feeble unless the developer 
contain a stronger acid than acetic acid. 

Sulphate of iron is a developer of great power, and may be 
employed with advantage when its action is correctly under- 
stood. Being a substance belonging to the mineral kingdom, it 
is not favorably constituted for producing that opacity of image 
which is characteristic of pyrogallic acid ; but on the other 
hand it is a much stronger reducing agent, and will bring out 
a perfect picture, when from some opposing conditions the or- 
dinary developer proves ineffectual. 

In order that an iodized collodion may be adapted for de- 
veloping with sulphate of iron, so as to give sufficient intensity 
at one operation, it ought to be of that kind which gives 
strong contrast of image. It must likewise be a collodion 
working clean and free from fogging, inasmuch as the sulphate 
of iron has a tendency to precipitate the silver in an irregular 
manner upon the film greater than that of the pyrogallic acid. 

The state of the nitrate bath must also be considered in 
making choice of a developer. Newly-prepared baths which 
yield rather faint negatives with pyrogallic acid, seldom suc- 
ceed with sulphate of iron ; the image becomes rapidly fogged 
and is useless. Baths made from fused nitrate of silver ; baths 
containing acetate ; and old baths contaminated with organic 
matter, can on the other hand be worked more successfully 
with sulphate of iron than with pyrogallic acid. The rule ap- 



NEGATIVE DEVELOPERS. 1Y9 

pears to be that if the solution is in a state for giving very i-ed 
negatives with great intensity and freedom from fogging, the 
inorganic developer will be the better of the two : for in that 
case the organic reactions of the film are already at their maxi- 
mum, and do not further need an organic developer like pyro- 
gallic acid. 

Depression of temperature is always nn indication for the 
use of sulphate of iron, and in such a case its superiority is 
especially evident. When the thermometer sinks to 40 deg. 
Fahr., it will be found that the ordinary solution of pyrogallic 
acid and acetic acid acts very slowly, and, in the case of col- 
lodion which has undergone a little organic decomposition, it 
does not bring out the dark shades effectually, so that on ex- 
amining the negative after fixing, it appears to have received 
an insufficient amount of exposure in the camera. An in- 
crease in the strength of the solution, using more of the pyro- 
gallic acid and less acetic acid, does not altogether remedy the 
defect, although it adds to the opacity of the parts which have 
received most light. The substitution of citric acid for the 
acetic would weaken the reducing power and be a positive 
evil. Nothing then remains but either to warm the bath and 
developing room by a stove, or to employ the sulphate of iron. 

The difficulty of securing a proper gradation of tone is espe- 
cially felt, not only in cold weather, but also when the picture 
embraces a variety of objects which contrast strongly in their 
power of reflecting light. Pyrogallic acid as a developer is 
apt either to destroy the definition in the light parts by pro- 
ducing absolute opacity of the negative, or to exhibit all the 
effects of overaction ol' light and i*ed solarization. In this way 
the folds of white drapery suffer, and the distance in land- 
scapes is lost. Such subjects can be photographed successfully 
either by using a feeble negative collodion destitute of organic 
reactions, or by developing with sulphate of iron. Monu- 
ments of white marble standing out against the sky, with 
cypress trees in the background, have been well copied by us- 
ing sulphate of iron ; and although equally good results may 
be obtained wdth pyrogallic acid, by giving a long exposure 
and working with a small aperture to the lens, yet this can 
only be expected when the bath and collodion are in the most 
perfect working order. 

There are other causes of imperfect gradation of tone which 
sulphate of iron is employed to remedy. When, for instance, 
the atmosphere is yellow or murky, and no clouds exist to 
throw back the light into the shadows, it becomes very diffi- 



180 THEOKY OF PHOTOGRAPHY. 

cult to work a simply iodized collodion successfully witli pyro- 
gallic acid ; the blacks of the nes^ative are too opaque, and the 
shadows too transparent. A longer exposure in the camera in 
this instance is only a partial remedy, because it invariably 
flattens the picture, destroying its rotundity and stereoscopic 
efi'ect. The use of sulphate of iron is exactly adapted to meet 
the case, for it has great power in developing weak radiations, 
whilst at the same time it precipitates the silver in a compara- 
tively metallic and translucent form. The resulting negative 
is therefore soft, and free from violent contrasts of light and 
shade. 

It has been said that the employment of sulphate of iron 
invariably shortens the necessary exposure in the camera, but 
this statement is incorrect. That it does so in a bad light and 
at a low temperature is certain, but probably if the experiment 
be made under opposite conditions, the same result will not be 
obtained. In the case of a sun-lit view, for instance, taken in 
the spring months, the writer finds pyrogallic acid abundantly 
strong enough to bring out the weaker radiations with a mini- 
mum of exposure, the collodion being supposed to contain only 
iodides. 

Acetate of iron has been used in photography ; it produces 
nearly as much density as pyrogallic acid, and at the same 
time is equal to the sulphate in its power of developing the 
shadows. A similar effect may be obtained by mixing the 
ordinary sulphate of iron with half its w^eight of crystallized 
acetate of soda. 

Effect of Varying the Mode of Development. — The remarks 
made in the last section, on the contrast in collodion positives 
as affected by the mode of development, apply also in the 
case of negatives. We have already seen that an ordinary 
iodized collodion may, when employed in the subdued light of 
a glass studio, produce a soft negative, and yet in a glare of 
sunshine it may yield an intense negative. If the intensity be 
greater than is desirable, it is in the power of the operator to 
remedy it in a measure by removing a portion of the free 
nitrate of silver from the surface of the film. To effect this, 
it will only be necessary to flood the plate with a large quan- 
tity of a diluted solution of pyrogallic acid containing perhaps 
half a grain of the reducing agent to the ounce of water ; or, 
more effectually still, to wash the plate with water, after ex- 
j)0sure, and then to develop it by the addition of a few drops 
of the bath solution to the pyrogallic acid. The picture thus 
obtained will have less contrast and solarization than before, 



NEGATIVE DEVELOPERS. 181 

and the developing action may be pushed far enough to bring 
out the deepest shadows, without adding too much to the in- 
tensity of the h'ghts. When the converse of the foregoing 
happens, and the image on a simply iodized collodion is de- 
ficient in contrast, it is recommended to increase the relative 
proportion of nitrate of silver by making an addition of that 
substance to the developer before applying it to the film. 

Developer for Bromo-iodized Collodion. — However mnch 
opinions may be divided as to the relative merits of pyrogallic 
acid and sulphate of iron as developers for a simply iodized 
collodion, no doubt exists that the latter is the more suitable 
for a bromo-iodized collodion. The great reducing power of 
the salts of iron is precisely what we require in the presence 
of bromide of silver, a salt which considerably retards reduc- 
tion ; and, as already shown, the fine red or black tone which 
pyrugallic acid imparts to the negatives is not seen when the 
collodion contains bromide. The tendency to produce fogging 
which the salts of iron exhibit in the case of a simply iodized 
collodion, is nearly absent when the collodion contains bro- 
mide. A subsequent development with pyrogallic acid will 
almost in every instance be required, which renders the opera- 
tion somewhat tedious, but it is amply compensated for by the 
increased vigor and contrast thereby obtained. 

Fixing Agents for Negatives. — Cyanide of potassium acts 
quickly in removing the iodide of silver, and the plates do not 
require much subsequent washing ; the film is also left in a 
favorable state for continuing the development with mixed 
pyrogallic acid and nitrate of silver when required. We rec- 
ommend, however, hyposulphite of soda in preference to the 
cyanide of potassium, as safer in the hands of a beginner. 
Negative images are more easily dissolved by fixing agents 
than collodion positives, and therefore, unless much care be 
exercised, the application of cyanide lowers the intensity ma- 
terially, and whitens the surface of the picture by converting 
it into cyanide of silver. This is especially the case in work- 
ing in the open air, and attempting to fix the image whilst the 
sun is shining upon the plate. Collodion negatives developed 
with sulphate of iron are less soluble in solution of cyanide of 
potassium than those in which pyrogallic acid is used as the 
reducing agent ; and the use of bromide in the collodion like- 
wise diminishes the solubility. 

Modes of Strengthening a Finished Impression which is 
too feeble to he used as a Negative. — The ordinary plan of 



182 THEOEY OF PHOTOGRAPHY. 

pushing the development cannot be applied with advantage 
after the picture has been washed and dried. In that case, if 
it is found to be too feeble to print well, its intensity may be 
increased by one of the following methods : 

1. Treatment of the Image with Sulphuretted Hydrogen or 
Sulphide of Ammoniurrh. — The object is to convert the me- 
tallic silver into sulphide of silver, and if this could be done, 
it would be of service. The mere application of an alkaline 
sulphide has, however, but little effect upon the image, ex- 
cepting to darken its surface and destroy the positive appear- 
ance by reflected light : the structure of the metallic deposit 
is too dense to admit of the sulphur reaching its interior. 

This may be obviated by first converting the image into the 
white salt of mercury and silver by the application of corro- 
sive sublimate, and afterwards treating it with a solution of 
sulphuretted hydrogen or sulphide of ammonium. Negatives 
produced in this way are of a brown-yellow color by trans- 
mitted light, and remarkably opaque to chemical rays. In- 
stead of the ammonium salt, plain ammonia diluted, cyanide 
of silver dissolved in cyanide of potassium, solution of hypo- 
sulphite of soda, and other substances may be employed. 

The employment of corrosive sublimate has one serious 
drawback, viz., the injurious effect which even a trace of a 
salt of mercury exerts upon the sensitiveness of iodide of 
silver ; hence if the glass plates are not cleaned with extra- 
ordinarj' care, or if the slightest portion of the mercury salts 
finds its way into the bath, injurious effects will follow. 
Some dispense entirely with the employment of the sublimate, 
and act on the image with a solution of iodine in iodide of 
potassium until it is converted into iodide of silver, after 
which the sulphide is applied in the usual way. The sulphide 
is, in fact, the principal agent in producing the intensity, and 
no other chemical is actually required, excepting for the pur- 
pose of rendering the ima"ge sufficiently porous to allow of a 
proper penetration by the sulphur. 

Another Process. — The image is converted into iodide of 
silver by treating it with a solution made by dissolving a grain 
of iodine in an ounce of water by the aid of a little alcohol. 
It is then washed — to remove the excess of iodine — exposed to 
the light, and a portion of the ordinary developing solution, 
mixed with nitrate of silver, poured over it. The changes 
which ensue are precisely the same as those already described ; 
the whole object of the process being to bring the metallic 



NEGATIVES STRENGTHENED. 183 

surface back again into the condition of iodide of silver 
modified by light, that the developing action may be com- 
menced afresh, and more silver deposited from the nitrate in 
the usual way. 

In a former edition a solution of iodine in iodide of potas- 
sium was recommended for the conversion of the image into 
iodide in this intensifying process. This method, however, is 
liable to fail, for if the solution be kept upon the plate until 
the whole image becomes yellow, the sensitiveness to light is 
in a great measure lost. At present, therefore, a weak aqueous 
solution of iodine seems preferable, the condition of the film 
so produced being analogous to that of the Daguerreotype, the 
metallic silver being acted on superficially, and never entirely 
converted into iodide. 




CHAPTER IX. 

ON THE THEOEY OF POSITIVE PRINTING. 

The subject of collodion negatives having been explained in 
the previous chapter, we proceed to show how thej may be 
made to yield an indefinite number of copies with the lights 
and shadows correct as in nature. 

Such copies are termed " Positives," or sometimes " Posi- 
tive prints," to distinguish them from direct positives upon 
collodion. 

There are two distinct modes of obtaining photographic 
prints : First, by development, or, as it is termed, by the nega- 
tive process, in which a layer of iodide or chloride of silver is 
employed, and the invisible image developed by gallic acid ; 
and second, by the direct action of light upon a surface of 
chloride of silver, no developer being used. These processes 
involving chemical changes of great delicacy require a careful 
explanation. 

The action of light upon chloride of silver was described in 
Chapter III. It was shown that a gradual process of darken- 
ing took place, the compound being reduced to the condition 
of a colored subsalt ; also, that the perfection of the change 
was increased by the presence of excess of nitrate of silver, 
and of organic matters, such as gelatine, albumen, etc. 

We have now to suppose that a sensitive paper has been pre- 
pared in this way, and that a negative having been laid in 
contact with it, the combination has been exposed to the 
agency of light for a suflficient length of time. Upon remov- 
ing the glass, a positive representation of the object will be 
found below, of great beauty and detail, l^ow if this image 
were in its nature fixed and permanent, or if there were means 
of making it so, without injury to the tint, the production of 
paper positive would be a simple department of the photo- 
graphic art ; for it will be found that with almost any nega- 
tive, and with sensitive paper, however prepared, the picture 
will look tolerably well on its first removal froin the printing- 
frame. Immersion in the bath of hyposulphite of soda, how- 
ever, which is essentially necessary in order to fix the picture, 



POSITIVE PRINTING. 185 

produces an unfavorable effect upon the tint, decomposing 
the violet-colored subchloride of silver, and leaving behind a 
red substance which appears to be united to the fibre of the 
paper. 

Other chemical operations are therefore required to remove 
the objectionable red color of the print, and hence the con- 
sideration of the subject is naturally divided into two parts ; 
first, the means by which the paper is rendered sensitive, and 
the image impressed upon it ; and secondly, the subsequent 
fixing and toning, as it may be termed, of the proof. 

The present chapter will also include, in an additional sec- 
tion, a condensed account of the most important facts relat- 
ing to the properties and the mode of preservation of photo- 
graphic prints. 

Section I. 

Printing the Proof. 

When a sheet of the photographically prepared paper is ex- 
posed to the light, we observe it to assume various colors, each 
one deeper and more intense than that which preceded it. 
These shades of color are not always the same, but vary more 
or less with the mode of preparing the paper, as will presently 
be shown. The sequence of tints, in the case of a paper pre- 
pared simply with chloride and nitrate of silver, is as follows : 
— Pale violet, violet-blue, slate-blue, bronze, or copper-color. 
When the bronze stage is reached, there is no further change. 

On immersing the paper, darkened as above described, in 
the fixing bath of hyposulphite, the violet tones are destroyed 
and the print assumes a red or brown color, which is most in- 
tense in the parts where the light has acted longest. There- 
fore we see, that, to produce a good photographic print, the 
negative must possess considerable opacity in the dark parts; 
for if it be pale and feeble, the light passes rapidly through it 
and darkens the paper universally, before the exposure has 
been sufiiciently prolonged to insure the requisite degree of 
reduction ; hence the deepest shadows of the resulting posi- 
tive are not dark enough, and there is a want of contrast which 
is fatal to the efiiect. A good negative should be so opaque as 
to preserve the lights of the printed image beneath clear, until 
the darkest shades are about to pass into the bronze or cop- 
pery condition. If the amount of intensity be less than this, 
the finest effect cannot be obtained. 



186 THEORY OF PHOTOGRAPHY. 

Let US now pass on to consider more carefully the exact 
function of each of the constituents of the sensitive sheet, and 
to show how the effect may be varied by altering their relative^ 
proportions, or by introducing substances not usually em- 
ployed. 

The printing process in its most simple form may be con- 
ducted as follows: Take pure Swedish filtering paper, free 
from size and other extraneous matters, and float it upon a 
solution of nitrate of silver containing about one hundred 
grains to the ounce of water; then dry and expose it to a 
strong sunlight. The darkening action will take place, but 
with such extreme slowness as at first to convey the impression 
that the paper is quite insensitive to light ; by perseverance, 
however, for three or four days, a pale-brown tone will be ob- 
tained. One cause of this ditficulty with the simply nitrated 
paper is that the nitric acid in the nitrate of silver retards the 
reduction, and the pure fibre of paper does not possess a suffi- 
cient affinity for oxygen to enable it to overcome the opposi- 
tion. It is possible, however, to counteract the nitric acid by . 
adding ammonia so as to produce ammonia-nitrate of silver ; 
this accelerates the change considerably, and a few hours' ex- 
posure to strong sunlight will then give the requisite opacity. 

Swedish paper, however, although prepared with ammonia- 
nitrate of silver, darkens very slowly, excepting in strong sun- 
light, and the photographer will find by experiment that a 
minute quantity of chloride of silver in the prepared paper 
will enable him to obtain the desired result in minutes instead 
of hours of exposure. Other insoluble salts of silver, such as 
the phosphate and citrate, render the prepared paper more ' 
sensitive than when it has been treated with a soluble salt of 
silver only. The part, therefore, which we assign to such 
insoluble salts is that of "accelerators" to the luminous 
agency, and of all the accelerators the chloride of silver ap- 
pears to be the most remarkable. 

The following experiment will prove instructive in further 
exhibiting the function of the chloride of silver in ordinary 
sensitive paper. Take a piece of ordinary bibulous paper and 
float it for an instant upon the nitrate of silver solution which 
photographers employ in printing; then blot it o£f and im- 
merse for flve minutes in a solution of common salt containing 
ten grains to the ounce. This paper, when freed from excess 
of salt by washing in distilled water, may be viewed as con- 
taining only chloride of silver in contact with the cellulose. 
On exposing it to the light it will be found to change rather 



POSITIVE PRINTING. 187 

quickly to a pale violet tone. At tliat point, however, the 
reducing action will be suspended, and when the fixing bath 
of hyposulphite of soda is brought to bear upon the image, it 
will nearly disappear in consequence of decomposition and 
solution. Pictures printed upon chloride of silver only would 
be altogjther wanting in contrast, consisting only of half-tints, 
without any depth of shadow. 

In a second experiment, take several strips of paper prepared 
as described for experiment Ko. l,and apply washes of nitrate 
of silver of various degrees of concentration, such as 5, 10, 20, 
40, 80, and 100 grains to the ounce of water. On drying the 
strips, and exposing them successively beneath the same nega- 
tive matrix, it will be found that the pictures become more 
and more vigorous in proportion as the nitrate of silver solu- 
tion increases in strength, but that beyond a certain point a 
further increase in the concentration of the nitrate of silver 
does not add to the eifect. 

Having performed the above experiments, we are prepared 
to conclude therefrom, — that the office of the free nitrate of 
silver is to furnish the material which composes the metallic 
part of the image, and so to give intensity ; the chloride mean- 
while accelerating the change and adding to the sensitiveness 
of the prepared paper. 

It remains now to consider the action of the organic sup- 
porting basis, and this is perhaps equally important with that 
of the other constituents, although in the present state of our 
knowledge it cannot be defined with the same precision. If 
we take a sheet of Swedish filtering paper and immerse it in 
cold nitro-sulphuric acid, in such a way as to wet only one- 
half of the paper, the surface after washing and drying will 
consist in part of cellulose and in part of pyroxyline. Now it 
has already been shown that pyroxyline is in a manner indiffer- 
ent to the salts of silver, and that chloride of silver supported 
by pyroxyline, behaves in the sun's ray much in the same man- 
ner as chloride of silver supported upon a glass plate : hence 
we should anticipate that if a sheet of paper were converted 
only partially into pyroxyline, and subsequently treated with 
salt and nitrate of silver, the two halves would behave differ- 
ently on exposure. This expectation is correct, and the ex- 
periment will show not only that the darkening is more de- 
cided upon the unaffected cellulose, but that the image is less 
dissolved by the fixing bath, and has a softer and more velvety 
shade of color. In the one case the fixed print is of a warm 
red, and tones in solution of chloride of gold to a fine purple- 



188 THEORY OF PHOTOGRAPHY. 

black; in the otlier, it is very faint and metallic, or, if ex- 
amined after toning, cold and slaty in aspect. 

We may therefore add to what we have before said, the fol- 
lowing statement, viz., that whereas the function of the chlor- 
ide is to impart sensitiveness in photographic printing, and that 
of the nitrate of silver to give intensity, the organic matter 
acts by brightening the color. The artist requires an image 
which after simply fixing shall be of a warm red tone, and thus 
be capable of yielding a full brown or black on subsequent 
treatment with the gold solution ; he will find by experience 
that organic compounds of silver in the paper, produced by 
adding albumen or similar substances to the salting bath, will 
afford him the means of obtaining these varied tones, and that 
without them the picture will lack richness of effect. 

The chemist may perhaps be disposed to inquire more par- 
ticularly how the organic substance acts ; but we must be 
guarded in answering this question, because it involves the 
consideration of a class of actions which belong to an obscure 
chapter of chemistry. It is known that many oxides and sub- 
salts of metals attach themselves in a peculiar way to animal 
and vegetable fibre, although the precise nature of the union 
is uncertain. These same oxides commonly exhibit an affinity 
for the coloring matters used in dyeing, and are known as 
" Mordants," because they bind the colors on to the cloth and 
fix them so that they resist the action of water. An ordinary 
"iron mould" is a familiar instance of this kind of action, the 
red stain upon the linen consisting of an oxide or subsalt of 
iron, adhering to the fibre. Organic substances saturated with 
bichromate of potash and exposed to light, furnish another ex- 
ample, for it has been shown that the bichromate becomes in 
such a case reduced to an oxide of chronium, which is a true 
mordant, although a feeble one. IS' ow in the process of pho- 
tographic printing we suppose that besides the formation of 
subchloride of silver, protoxide of silver is reduced to the state 
of a lower or sub-oxide, and that this sub-oxide combines with 
the cellulose. Further, the fact of the cellulose or other or- 
ganic matter having an affinity for a sub-oxide facilitates the 
formation of that substance, and enables it when formed to 
withstand the action of bodies like hyposulphite of soda, which 
are known to possess the property of decomposing sub-oxide 
of silver when existing in an uncombined state. 

If photographic printing can be shown to bear any analogy 
to the operations employed in the art of dyeing, it would be 
anticipated that certain kinds of fibre would exhibit the affinity 



POSITIVE FEINTING. 189 

for the mordant oxide more completely tlian others. It is well 
known, for instance, that woolen stnffs take certain dyes with 
more facility than materials made of linen or cotton, so that if 
a cloth be woven partly of wool and partly of cotton, a color 
may be fast upon the former, but removable by washing from 
the latter So in photography we find that cotton immersed 
in nitrate of silver is less readily affected by the sun's rays than 
wool or silk. In speaking of the use of animal substances in 
printing, we must not indeed lose sight of the fact that these 
tissues invariably contain traces of chloride and also of phos- 
phate. The pure animal fibre, however, is believed to play an 
important part in the process, quite independently of any 
inorganic salts. This action of the animal matters we now pro- 
ceed to consider further. 

Swedish paper, prepared with chloride and nitrate of silver, 
although sufficiently sensitive, could not be used in photog- 
raphy. The picture would exhibit a complete want of defini- 
tion, and would also appear to be sunk in the substance of the 
paper so as to be seen more distinctly by transmitted than by 
reflected light. If the salt were employed in anything like 
quantity, chloride of silver would form in loose flakes upon 
the surface, and would burst out and fall away into the bath. 
The sizing of photographic paper has undoubtedly a mechani- 
cal action, keeping the chemicals upon the surface, and thus 
securing an even layer of chloride of silver in a state of ex- 
cessive division. This division, as already shown, is not the 
sole use of the sizing, for when it consists of albumen or gela- 
tine, it communicates a fine red tint to the image, and gives 
what artists term an aspect of transparency to the whole pic- 
ture. 

If we examine the action of the animal sizing chemically, 
we find that albumen, casein, and gelatine all withdraw from 
the bath large quantities of nitrate of silver, so that the solu- 
tion becomes continually weaker. A sheet of transparent gela- 
tine, allowed to swell up by imbibing a solution of nitrate of 
silver of the strength of twenty grains to the ounce of water, 
will appropriate nearly the whole of the dissolved nitrate, so 
that the liquid expelled from it by squeezing will yield less 
than three grains of silver to the ounce. The gelatine forms 
with the nitrate of silver a compound which may be designated 
gelati no-nitrate of silver, and which is highly photographic 
and colorific. Chloride of silver thrown down in presence of 
this gelatino-nitrate, does not clot together in the same manner 
as the pure chloride of silver, but exists in a state of excessive 



190 THEORY OF PHOTOGRAPHY. 

division, and remains for a long time without subsiding to the 
bottom of the liquid. Exposed to light, the gelatino-nitrate of 
silver darkens to a ruby-red color ; and chloride of silver pre- 
cipitated from an aqueous solution of the gelatino-nitrate 
never assumes, in the sun's rays, the slate-blue color characteris- 
tic of subchloride of silver, but changes quickly to a chocolate- 
brown tone. In the case of a pa]3er sized with albumen or 
gelatine, and subsequently salted and rendered sensitive, the 
action of the light is evidently compound, for the chloride de- 
composes at the same time that the oxide of silver is reduced 
by the animal matter. The behavior of the darkened paper 
always varies with the proportion which the chloride bears to 
the organic substance ; when the former is relatively large, the 
print exhibits the violet shades on its removal from the frame, 
and dissolves considerably in the fixing bath ; but with the or- 
ganic matters in excess, the color of the print is brick-red from 
the very first, and the tones are less affected by the solvent 
action of the hyposulphite of soda. 

FURTHER OBSERVATIONS ON THE PRINTING OF PHOTOGRAPHIC 

PROOFS. 

In considering the process of j)Ositive printing more care- 
fully, we may divide it as follows: The paper; salting bath; 
sensitizing solution ; various kinds of organic matter. 

The Paper. — The quality of paper sold for photographic 
purposes is variable, and often inferior, A difference exists 
in the length and thickness of the fibre of various kinds of 
cellulose, so that the resulting paper may be either coarse or 
smooth grained. Its smoothness cannot be estimated until 
the print has passed through all the processes of fixing and 
washing, because by hot pressing and other appliances it is 
easy to get up a fictitious glaze. 

Supposing the paper to be properly made in the first in- 
stance, yet much will depend upon the perfection of the pro- 
cess adopted for sizing. The defects which occur in a paper 
badly sized are of the following kind : First, portions of the 
finished pictures are pale in color, and have a spotty appear- 
ance, due to an inequality in the imbibition of liquid by the 
paper; some parts of which being comparatively impermeable 
by the nitrate of silver, and others more porous, the surface is 
unequally sensitive, and darkens in an irregular manner. 
Secondly, when the sizing is very soft, the chemicals sink too 
deeply into the paper, and the proofs are what is termed mealy 



POSITIVE FEINTING. 191 

and ineffective. Albumen, even when employed without any 
addition of water, gives very little glaze upon a paper of this 
kind, and no surface vigor can be obtained ; the sheets are 
often very tender, so that they become torn in the many wash- 
ings to which the photographic proofs are necessarily sub- 
jected ; and when albumen is used there is a strong tendency 
to a superficial blistering in the fixing bath, or in the washing 
waters, inasmuch as the alkaline solutions used in toning tends 
to lessen the tenacity of the size. Thirdly, paper may be too 
strongly sized, and when such is the case the amount of gloss 
given by albumen is considerable, but the prints are not easily 
toned in the gold solutions, and are fixed with difiiculty. 

There are too principal modes of sizing paper : first, with a 
mixture of starch and resin partially saponified by an alkali ; 
and second, with gelatine hardened by alum. The first me- 
thod is principally practiced on the Continent, and the latter 
in the papers of home manufacture. Papers sized with starch 
and saponified resin have necessarily an alkaline reaction, 
whilst the gelatine-sized papers exhibit an acidity due to the 
alum. In each case the paper improves more or less by keep- 
ing, because the size becomes gradually harder, and the sheets 
are, in consequence, less easily torn in the washings. Opin- 
ions are divided as to which mode of sizing is to be preferred, 
but the general impression is that the starch offers more me- 
chancal advantages when albumen is to be used in the salting 
solution, whereas the gelatine size gives a better surface layer 
of chloride of silver in the case of plain salted paper prepared 
without albumen. Towgood's paper is held in much esteem 
for printing by the ammonio-nitrate process, and probably 
answers better than any other in the market, but it is not well 
adapted for albumenizing. Papier Rive, so called from the 
place where it is manufactured in France, is a starch-sized 
paper, with a hard, smooth surface, not easily permeable by 
liquids ; hence it takes a high gloss on the albumen. It is 
somewhat rotten in texture, and apt to tear in the washing, 
but it assumes a fine brilliant color in the toning-bath, and is 
therefore much used for carte-de-visite and stereoscopic prints. 
The German paper, usually called Saxe, also starch-sized, is 
stronger in texture than the Pive, and well fitted for large 
photographs. From its more porous texture, it does not as- 
sume so high a gloss on the albumen as the Rive. 

The photographic properties of the paper are much affected 
by the mode of sizing adopted, even when albumen is after- 
wards used, for the picture is probably formed partly upon the 



192 THEOKY OF PHOTOGRAPHY. 

albumen and partly in the sizing. English papers tend to give 
red tones which become brown or chocolate-colored in the 
finished print. This is due in part to the use of gelatine, 
which, as before shown, forms a compound with nitrate of sil- 
ver, darkening in the sun to a ruby-red color, but in part to the 
alum employed to harden the gelatine, since alum is an acid 
salt, and acids tend to impart a foxy-red tone to the image. The 
foreign papers sized with starch or resin, produce tones which 
are of a sepia-brown after fixing, and of a purple-black when 
treated with the solution of gold ; the reason is partly because 
the starch and resin do not, like gelatine, exert a very marked 
action in reddening the picture, and partly because the sizing 
bas an alkaline reaction, and alkalies are found to diminish red- 
ness, just as acids increase redness. 

Many additional observations might be made on the color of 
the image yielded by the various commercial qualities of pho- 
tographic pa])er, but the above general division into gelatine- 
sized paper and starch-sized paper will be sufficient. At the 
same time, it should be borne in mind that the manufacturer 
has it in his power, by adding small quantities of organic bod- 
ies to the size, to modify the tone given by the paper, even 
when employed in the albumenized state. Hence, although 
the general characters of a paper may be those of a starch-sized 
paper, yet the print may assume a brick-red color in place of a 
violet tone, in consequence of some addition made to the size. 

Rendering the Paper Sensitive. — Under this head we speak 
first of the salting solution, and second of the nitrate bath. 
Observe, in the first place, that the strength of both solutions 
should be properly adjusted, so that when the amount of chlor- 
ide of silver in the paper increases, the excess of free nitrate of 
silver may increase also. Theoretically, three parts by weight 
of nitrate of silver will precipitate nearly one part by weight 
of salt, and a slight excess of the nitrate of silver will remain. 
These proportions, however, are not always adhered to, because 
in photographic printing there are many disturbing conditions. 
The sizing of the paper and the albumen glaze, appropriate a 
quantity of free nitrate of silver, as already shown ; and not 
only so, but the proportions of nitrate and of chloride left in 
the paper vary with the method of applying the solutions. 
There are three modes of spreading solutions on photographic 
paper — by brushing, by floating, and by total immersion. The 
first leaves a small, and the third a large amount of solid mat- 
ter ii|)on the paper, whilst the second gives a variable result 
according to the length of time the paper is left fioating upon 



POSITIVE FEINTING. 193 

the bath. Experience shows that for an albiiuienized paper, 
both solutions being applied by floating, the nitrate bath should 
be about six times as strong as the salting bath, and should be 
left twice as long in contact with the paper. In the case of a 
paper floated upon the salting bath, and brushed with the ni- 
trate of silver, the latter may be twelve times stronger than 
the former ; and when the papers are salted by total immersion, 
and sensitized by brushing, the salt may conveniently be 
reduced to a fifth of the ordinary weight, and the nitrate of 
silver left as before, so that the proportions may be nearly as 
thirty of nitrate to one of salt. 

The strength of the nitrate bath in photographic printing 
must also be regulated partly by the mode of sizing adopted. 
A hard-sized English paper keeps all the chemicals compara- 
tively upon the surface, and does not require such concentrated 
solutions as a paper which is more soft and spongy. Also, when 
any organic matter, like gelatine or albumen is added to the 
salting bath, the amount of salt must be lessened, because the 
glutinous character of such fluids causes more to be retained 
upon the surface of the paj)er. The difference in the atomic 
weights of the various soluble chlorides used in salting must 
also be borne in mind ; 100 grains of chloride of ammonium 
contain as much chlorine as 109 grains of chloride of sodium, 
or as 228 grains of chloride of barium (see the vocabulary). 

The Salting Bath. — The foregoing remarks apply both to 
the saline and to the sensitizing bath ; in those which follow, 
the two will be considered separately. The sensibility of 
photographic paper is regulated up to a certain point by the 
amount of salt used in its preparation. The quantity of alka- 
line chloride determines the amount of chloride of silver ; and 
with a 23roper excess of nitrate of silver, papers are, up to a 
certain point, more sensitive in proportion as they contain 
more of the chloride. Highly sensitized papers darken rap- 
idl_y, and pass very completely into the bronze stage. Those 
containing less chloride darken more slowly, and do not be- 
come bronzed with the same intensity of light. A photo- 
graphic print formed upon a paper very highly salted and 
sensitized, is usually vigorons, with great contrast of light and 
shade, particularly so when the printing is conducted in a 
strong light. Hence it will be an advantage, with a feeble 
negative, and in dull weather, to double the ordinary quantity 
of salt ; whereas in the case of an intense negative, and with 
direct sunlight, the deep shadows will be too much bronzed 
unless the quantity of chloride of silver in the paper is kept 



194 THEORY OF PH(JTOGRAPHY. 

low, SO as to stop the action of the strong lights at a certain 
point, and thus to allow the feeble ra3^s time to come forward. 

With regard to the effect which the amount of chloride in 
the paper exerts upon the color and general appearance of the 
print, the following statements may be made : Highly salted 
and sensitized papers give a picture more nearly approaching 
to black than those which, containing a small proportion of 
chloride of silver, are less sensitive to light. Hence, in print- 
ing upon paper weakly sensitized, in order to bring out the 
finer details of a highly intense negative, we find the image 
unusually red after lixing, and of a brown or mulberry color 
when toned. 

It is possible to carry the proportion of salt too far in photo- 
graphic printing; in that case, even though the excess of 
nitrate be properly maintained, the print appears cold and 
dull, because the chloride of silver is in too large quantity 
with reference to the organic matter. A reduction in the 
amount of salt, on the other hand, simply leaves the image of 
less contrast, but does not destroy its velvety softness, trans- 
parency and warmth depending upon the organic matter, and 
not upon the chloride. 

The JS it rate Bath. — The compound on which a positive 
print is formed is a chloride, or an organic salt of silver, with 
an excess of nitrate of silver ; hence nothing is gained by 
increasing the proportion of chloride of sodium, unless at the 
same time an addition be made to the quantity of free nitrate 
in the sensitizing bath. 

Let us consider more minutely the appearances which pre- 
sent themselves when the nitrate bath is too weak. If a 
sample of the ordinary salted and albumenized paper be floated 
for two or three minutes upon a solution of nitrate of silver 
of the strength of twenty grains to the ounce of water, the 
quantity of nitrate left upon the surface will be insufficient, 
and the following defects will appear: The paper darkens on 
exposure, but it does not reach the bronzed stage, the action 
appearing to stop at a certain point. On placing the print in 
hyposulphite of soda it becomes very pale, and, when tinted, 
looks cold and slaty, without depth of shadow. In almost all 
cases there exist on pictures of this kind large spots or patches 
of a paler color than the surrounding parts, since the capil- 
larity of the paper is unequal, and some portions absorb slowly. 
The spots are most abundant at that Q(\^<d of the paper which 
is uppermost in drying, and are nearly absent at the lower 
part where the excess of liquid drains down and becomes con- 



POSITIVE PRINTING. 195 

centrated by evaporation. In a second experiment the salted 
paper may be left as long as ten or fifteen minutes upon the 
same weak bath : the result will be improved thereby, for the 
albumen is allowed time to draw to itself more of the nitrate 
of silver. A third experiment may consist in dissolving 40 
grains of nitrate of silver in an ounce of water, and floating 
portions of the same paper upon it : the shadows will be deeper 
than before, and the color warmer, but in all probability there 
will be room for further improvement, since a 40-grain bath is 
scarcely strong enough for the foreign papers albumenized with 
a 10 -grain salt solution, although sufficiently so for the English 
albumenized paper of the same strejigth. Lastly, dissolve 60 
grains, 80 grains, and a 100 grains of nitrate of silver in 
three separate ounces of water, and float upon each, when it 
will be found that the pictures are all good, and differ very 
little in appearance. If, however, the time of floating be re- 
duced to a single minute, the nitrate bath of a 100 grains will 
prove the best; and some artists consider that, in photographic 
printing, both time and money are saved by employing a 
highly-concentrated nitrate bath, and floating the papers upon 
it for not more than a minute. The defect which the writer 
would apprehend under such circumstances would be a tenden- 
cy to spottiness from uneven absorption by the paper, since a 
lengthened floating is certainly favorable to even precipitation 
of the chloride. 

Those precautions which are observed in making the nitrate 
bath for collodion negative photograjihy, are unnecessary in 
the case of the bath for printing. We saturate the former 
bath with iodide of silver, but the printing bath need not be 
saturated with chloride of silver, since this compound, although 
not absolutely insoluble in solution of the nitrate, dissolves in 
a proportion so small, that it may be disregarded. I^either is 
it actually necessary to examine the crystals of nitrate of sil- 
ver for free nitric acid, for unless the sample of nitrate be very 
impure, the retarding effect of the nitric acid will be inapprec- 
iable, and especially so in the case of albumenized paper, which 
possesses usually a slightly alkaline reaction. 

A nitrate bath containing free oxide of silver, however, and 
giving an alkaline reaction to litmus, would in some cases be 
injurious, since alkaline nitrate of silver does not properly co- 
agulate albumen, and in consequence a bath of this kind soon 
exhibits a white turbidity when the papers are floated upon it. 
There are certain qualities of albumenized paper sold in com- 
merce, which tend to precipitate the same white substance in 



196 THEOKT OF PHOTO&KAPHT. 

the sensitizing bath, and the writer believes this to be due in 
part to alkalinity. The strength of the bath, however, must al- 
ways be noted ; for the weaker it becomes, the greater the tend- 
ency to dissolve away the albumen without coagulating it. In 
such a case the greater part of the nitrate of silver is converted 
into chloride, and not being properly retained upon the surface 
of the paper by the coagulated albumen, falls away into the so- 
lution. 

Papers rendered sensitive upon a nitrate bath faintly acid 
with nitric acid, or with acetic acid, are less liable to spontane- 
ous reduction in the dark : whereas papers prepared upon a 
bath which has become alkaline from continued employment 
of an alkaline albuminized paper, or other causes, soon change 
on keeping. 

Acetate of silver exercises an important effect in the photo- 
graphic negative bath, but in the bath for printing, its action 
is not very remarkable. The only perceivable difference is a 
little extra bronzing in the shadows, and an increased difficulty 
of keeping the paper without discoloration in the dark. Or- 
ganic matter is also mentioned as greatly influencing the action 
of the negative nitrate bath, but in the printing bath its effect 
is inappreciable, and even when the bath has become highly 
colored by the albumen, its action is nearly the same as at first. 
The brown coloration above alluded to is probably due to the 
gradual formation of a sub-albuminate of silver, partially sol- 
uble in solution of nitrate of silver. 

Before leaving this subject, we must advert for a moment 
to the employment of the compound known as ammonia-nitrate 
of silver* in photography. The advantage derived from its 
use is an increase of sensitiveness, and also of intensity in the 
image. Another advantage of the ammonia-nitrate is that the 
color of the print is improved, the redness being diminished 
and a soft velvety aspect being given to the image such as 
would be difficult to secure, on plain paper, with simple nitrate 
of silver. 

There are, however, disadvantages attending the use of am- 
monia-nitrate of silver, which prevent it from being generally 
adopted. In the first place it does not coagulate albumen, so 
that albumenized paper floated upon ammonia-nitrate of silver 
loses its surface-varnish, and appears dead, like the plain 
paper. Secondly, it is more liable to spontaneous change, and 
to discoloration by traces of organic matter, than simple nitrate 

*The chemistry of ammonia-nitrate of silver is explained in the vocabu- 
lary. 



POSITIVE PMNTING. 197 

of silver ; and hence when used as a bath, it becomes perfectly 
black in the course of a few days, from sizing, etc., dissolved 
out of the salted paper. Thirdly, the action of the salted 
paper upon ammonia-nitrate of silver liberates free ammonia, 
as will be seen by referring to the vocabulary, page 75, and 
and this free ammonia being a solvent of chloride of silver, 
attacks the sensitive coating and dissolves it, thus producing 
white lines and transparent markings. The latter objection is 
the most formidable of all, and in consequerce of it the am- 
monia-nitrate has nearly fallen into disuse, excepting in the 
case of papers purposely prepared with a very small quantity of 
salt, so as to avoid the production of free ammonia as far as is 
possible. Such papers are economical, because a compara- 
tively weak silver solution suffices to sensitize them, and to 
give the requisite vigor to the shadows. The color of the 
finished picture, however, on ammonia-nitrate paper feebly 
salted, is not black, but rather of a chocolate-brown, since the 
diminution in the quantity of chloride increases redness, and 
the effect of the ammonia-nitrate of silver in an opposite di- 
rection is not sufficient to counteract this tendency. 

Organic Bodies used in Printing. — The most important 
of these is albumen. Albumen is remarkable for producing a 
smooth and homogeneous layer upon the very surface of the 
paper, so that every detail of the negative is rendered with an 
amount of distinctness which cannot be obtained in any other 
way. The prints are clear and brilliant, retaining even when 
dry much of the transparency which plain paper pictures ex- 
hibit only whilst in the water. 

The main obstacle to the general adoption of albumen is 
the difficulty of applying it evenly to the surface of the paper. 
Being a glutinous fluid, and not immediately amalgamating 
with the sizing, it is apt to run into lines when the paper is 
floated only for a short time, such as a few seconds ; whilst on 
the other hand, if the paper be left for several minutes 
upon the albumen, a portion of the size is dissolved, and the 
albumen in consequence sinks into the paper, and does not 
impart the proper amount of gloss. In order to understand 
this, we m.ust bear in mind that albumen is not a neutral fluid, 
but possesses an alkaline reaction, due to the presence of a 
small quantity of soda; hence, on adding chloride of ammo- 
nium to albumen a development of free ammonia takes place, 
easily perceptible to the smell, and ammonia is a solvent of the 
materials used in sizing paper. To overcome this difficulty 
of applying the albumen, and to obtain a greater amount 



198 THEORY OF PHOTOGRAPHY. 

of gloss, makers of albumenized paper have found it 
advantageous to expose the albumen in an open ves- 
sel to the air, until a considerable amount of evapor- 
ation has taken place. This evaporation does not always 
render the albumen more glairj; on the other hand, the 
fluid often becouies gradually more limpid, acquiring a rather 
offensive odor, and an acid reaction to litmus paper. Albu- 
men so prepared runs upon the paper very easily, and does 
not dissolve the size, but it possesses some objectionable quali- 
ties, to be pointed out under the head of " toning." That 
defects should arise from decomposition of albumen, is not to 
be wondered at when we consider that one of the constituents 
of this substance is sulphur, which during the putrefaction 
passes into the state of sulphuretted hydrogen, and is the cause 
of the offensive smell. Tiie nitrate bath is soon rendered tur- 
bid by the use of stale albumenized paper, and the sensitive- 
ness to light is injured, so that the half-tones of the picture do 
not appear until after a prolonged exposure. In preparing a 
highly albumenized paper, the requisite amount of evaporation 
may be effected over hot water at a temperature short of the 
coagulating point of albumen ; or what in most cases will be 
found sufficient, the wet albumenized sheet may be rapidly 
dried by an ascending current of hot air. Pressing between 
heated rollers will still further condense the finished sheet, and 
give it the appearance of possessing a hard and glossy surface. 

The reddening action of gelatine, although greater than that 
of starch, is less than that produced by albumen, and the sur- 
face brilliancy is also less, Caseine, the animal principle of 
milk, gives good definition, and a red color like albumen, but 
is destitute of gloss. Were it not for the difficulty of prepar- 
ing soluble caseine, it would probably come into more ex- 
tended use, since the tone, when modified by a deposit of 
gold, is very agreeable. Serum of milk is a convenient form 
of employing a dilute solution of caseine ; since the rennet 
used in coagulating the milk does not separate the whole of 
the caseine, but leaves a little dissolved in the liquid ; by agi- 
tation with white of eggs, and subsequently boiling, the sus- 
pended oil globules may be entangled, and the serum thus 
clarified. 

Citrate and tartrate dissolved in the salting bath exercise an 
effect upon the color of the print quite as remarkable as that 
of albumen. Paper prepared with citrate, in addition to 
chloride of silver, darkens to a fine purple color, which be- 
comes brick-red in the fixing bath. Oxalate, however, has not 



POSITIVE p'einting. 199 

the same action ; paper prepared with oxalate and chloride of 
silver darkening to a violet-bhie color, reseniblino; that of the 
ordinary subchloride. These facts will enable the reader to 
understand the remarks previousl_y made on the composition 
of materials for sizing paper ; and to see that the maker might, 
if so desired, introduce small quantities of organic substances 
capable of modifying the color and general aspect of the print. 

ATTiount of Silver in Sensitive Sheets. — To determine this 
point roughly, fifty whole sheets of Saxe paper, 18x22, albu- 
minized with nearly pure albumen, containing ten grains of 
salt to the ounce, were floated for three or four minutes each, 
on a fifty-five grain nitrate bath measuring 137 ounces. The 
sheets were found to remove 17 ounces of liquid from the 
bath, and to impoverish the remainder to the extent of five 
grains of nitrate of silver per ounce. The whole quantity of 
nitrate of silver absorbed by each sheet must, therefore, have 
been about half a dram. In a second experiment, a quarter 
sheet of albumenized paper Saxe was dried and weighed ; it 
was then floated for four minutes upon a forty-five grain solu- 
tion of nitrate of silver, dried and weighed a second time ; 
the gain in weight amounted to seven grains. 

Preservation of Sensitive Sheets. — The discoloration of sen- 
sitive paper in the dark is due to a slow reducing action of 
organic matter upon the free nitrate of silver. The rapidity 
of the change varies much with the nature of the organic mat- 
ter, but more so with the state of the nitrate of silver. Al- 
kalies in the size of the papers or in the sensitizing bath, 
facilitate the discoloration, whilst free acid and acid salts, like 
alum, etc., retard it ; hence French papers darken more 
quickly than English papers as a rule, and ammotiio-nitrate 
paper is more unstable than that prepared with simple nitrate 
of silver. 

The degree of dryness of the atmosphere in which the 
papers are kept also effects their rate of change, since mo'&' 
ture appears essential to the reduction. Hence the various 
contrivances devised for drying the paper, the best of which 
appears to be that of Mr. Spiller, in which a box is constructed 
with a false bottom pierced by holes, and lumps of quick- 
lime are placed beneath. To all such plans, however, 
there are some objections, for it has been found that papers, 
when rendered absolutely dry, do not darken vigorously in 
the light, and that it is necessary, in consequence, to leave 
them for a short time in close proximity to a damp cloth, un- 



200 THEORY OF PHOTOGKAPHT. 

til the requisite amount of water has been absorbed. Others, 
again, have observed that, although sensitive papers, kept for 
many months in a drying box, may print to a sufficiently deep 
shade, yet that the image does not tone in the alkaline gold 
bath so readily as a print upon newly-sensitized paper. 

Section II. 

Toning the Proof. 

By the terra "toning" we understand the removal of the 
reddish color described in the first section as the proper tint 
of a photographic positive, after the compounds of silver un- 
acted upon by light have been extracted by an appropriate 
fixing bath. Colors approaching to brick-red are in them- 
selves so unartistic that from the very first discovery of pho- 
tography means were tried of removing them ; and the em- 
ployment of sulphur was the plan originally adopted for that 
purpose. 

TONING BY SULPHUR. 

It is well known that articles of silver-plate become dark- 
ened by exposure to the fumes of sulpliur, or to those of sul- 
phuretted hydrogen, of which minute traces are always present 
in the atmosphere. If the stopper of a bottle of sulphuretted 
hydrogen water be removed, and a simply fixed photographic 
positive suspended over it, the picture will lose its characteris- 
tic red tone and become nearly black. The black color is 
even more intense than an experienced chemist would have 
anticipated, because analysis teaches us that the actual quan- 
tity of silver present in a ]3hotographic picture on paper is 
infinitesimally small, and it is well known that sulphide of 
silver, although of a deep brown color, approaching to black, 
when in mass, exhibits a pale-yellow tint in thin layers, so 
that a mere film of silver converted into sulphide possesses 
very little depth of color. To explain the difficulty, it has 
been suggested that the toning action of sulphur on a red 
print is probably due to the production of a sub-sulphide pos- 
sessing an intense colorific power, like the sub-oxide and sub- 
chloride of silver. When the toned picture is subjected to 
the further action of sulphur, it is converted into the ordinary 
protosulphide of silver, and becomes yellow and faded. 

It is not necessary to enter into any details of the various 
processes originally recommended for toning prints by means 



TONING THE PEOOF. 201 

of sulpliiir. The principle was the same in all, but the mode 
in which tlie sulphur was set free, and brought to hear upon 
the proof varied. Commonly, a solution of hyposulphite of 
soda was mixed with some chemicals which gradually decom- 
posed it. Everything requiring explanation under this head 
will he treated in the next section, when we speak of the 
properties of the hyposulphite fixing bath. 

TONING BY GOLD. 

After the sulphur toning process has been discarded, the 
salts of gold were used for improving the normal color of the 
paper photograph, and the merit of first introducing them is 
due to M. Le Grey, of Paris. These methods, being still in 
vogue. Mall require a detailed description. 

The Single Fixing and Toning Bath. — A simple mode, and 
one even now sometimes followed, is to add chloride of gold to 
the ordinary hxingbath of hyposulphite of soda. The prints are 
immersed in the resulting liquid immediately on taking them 
from the frame, and the lirst action of the " fixing and toning 
bath " is to dissolve out the unchanged silver salts, and to 
. leave the image of the usual red color Shortly, however, the 
red color begins to pass gradually into blue or black, and the 
toning is complete. 

At lirst sight the above process appeared correct both as re- 
gards theory and practice. A more extended experience, how- 
ever, led to its condemnation, for it was found that the im- 
provement of the color was not due to a simple precipitation 
of gold upon the surface of the image, as had been at first sup- 
posed, but partly to a deposit of gold, and partly to a com- 
mnnication of sulphur. When a solution of chloride of gold 
is added to hyposulphite of soda, we have in the liquid not 
only the double hyposulphite of gold and soda known by the 
name of Sel d'Or, but also a portion of the unstable tetrathion- 
ate of soda, prone to liberate sulphur. The action of the 
bath is therefore complex from the very first, but becomes 
more so on keeping the solution for a time, since spontaneons 
decomposition ensues, as will be more fully shown in the next 
section. Practical photographers were not slow in discovering 
that the fixing and toning bath was inconstant in its action, 
and that although the prints were colored with as much 
rapidity in an old as in a newly-mixed bath, yet that the tints 
were more fugitive in the former case than in the latter. The 
reason was that whereas the newly-prepared bath acted mostly 



202 THEORY OF PHOTOGRAPHY. 

by depositing gold, and only partially by communication of 
sulphur, the old bath, on the other hand, toned the prints by 
sulphuration. The reader will doubtless be surprised to hear 
that the colors produced upon the photographic picture by 
means of gold closely resemble those obtained by sulphur ; 
yet such is the case, for although shades of blue are 
characteristic of gold, and shades of brown of sulphur, yet 
it requires a practised eye to distinguish between them, and 
prints toned by sulphur often possess the line purple-black 
which many suppose to be due to a deposit of gold. 

In the gold toning process, as now employed, all danger of 
sulphuretting the prints is avoided. Hyposulphite of soda 
having been proved to suffer decomposition in presence of 
chloride ot gold, we no longer mix the two solutions but pre- 
fer to tone the picture first, and fix it subsequently. The 
coloring action of a simple solution of chloride of gold upon 
the photographic image may be thus explained : — The 
chlorine previously in combination with gold passes to the re- 
duced silver salt, bleaching the lighter shades by converting 
them into the white protochloride of silver, and imparting to 
the shadows a deep violet tint due to the production of sub- 
chloride of silver; at the same time metallic gold is deposited, 
but its effect is not very clearly seen at this stage of the pro- 
cess, since a deep violet color of nearly equal intensity may be 
obtained by using a solution of chlorine in place of chloride of 
gold. If, however, the tv»ned proof be acted upon by a fixing 
bath of hyposulphite of soda, all that portion of the violet 
color which depends upon sub-chloride of silver will be des- 
troyed, since the sub-chloride is decomposed by fixing agents. 
Another portion of the color will resist the action of the hypo- 
sulphite, and this is j)robably metallic gold. 

During the whole process of toning in solution of chloride 
of gold we observe a gradual lowering of intensity in the 
image ; it has been found that this destructive effect is greater 
when the solution of the chloride contains free hydrochloric 
acid, and that it is lessened by the addition of an alkali; 
hence the employment of alkaline solutions of chloride of gold, 
first introduced into this country by Mr. Waterhouse, of 
Halifax, must be deemed an improveiTient. 

The question as to the manner in which the alkali acts in 
preventing the image from being eaten away has been vari- 
ously answered. The first effect of adding carbonate of soda 
to chloride of gold is to neutralize the free hydrochloric acid 
which that compound invariably contains, and thus to produce 



TONING BY GOLD. 203 

a double chloride of gold and sodium. Now, it has been sug- 
gested that this chloride of gold and sodium is the real toning 
agent ; that the process consists essentially in the substitution 
of gold for silver; and that a double chloride of gold and 
sodium becomes a double chloride of silver and sodium, metal- 
lic gold being at the same time thrown down, and taking the 
place of the silver of the proof. Further, it has been urged 
that in the alkaline toning process lies the real key to the pro- 
duction of permanent photographic pictures, since the image 
of silver, proved to be unstable, is by it converted into an 
image of gold, not prone to change. These views are in- 
genious, but it is doubtful whether they express the truth ; 
for, on subjecting the image to the action of chloride of gold, 
we do not find that an atom of silver is dissolved for each 
atom of gold deposited, whilst as regards the entire conversion 
of the image into metallic gold, it may be shown to be 
impracticable. MM. Davanne and Girard have suggested 
that the alkaline addition to the chloride of gold is useful in 
neutralizing traces of free nitric acid, always liberated when 
the organic matter reduces the nitrate of silver in the process 
of printing. Undoubtedly the positive, on its removal from 
the printing-frame, has an acid reaction, and the carbonate of 
soda in the toning bath must operate beneficially in neutral- 
izing it. This, however, is not the sole use of the alkali, 
according to the writer's experience ; it probably acts, also, by 
converting a portion of the chloride of gold into an oxide of 
gold, which is an unstable substance, and may be made to 
yield up metallic gold to the proof. 

When carbonate of soda is added to chloride of gold the 
change into oxide is at first only partial, and an oxychloride 
of gold is probably formed. The oxide of gold, or auric acid, 
does not appear of itself to possess the power of toning the 
picture ; and hence the total conversion of the chloride under 
the infiuence of the alkali renders the bath useless to the pho- 
tographer. If, however, a portion of the gold still remains in 
the form of .chloride, and the toning action can once be 
started, a spontaneous decomposition of the oxide of gold will 
take place by catalysis, and a larger quantity of gold will thus 
be thrown down without the same injury to the print from 
communication of chlorine. These views may appear com- 
plex, but they have been adopted by the author after a care- 
ful study of the properties of the alkaline aurates. 

The alkaline substance usually employed for admixture 
with chloride of gold is the carbonate of soda, but it has 



204 THEORY OF PHOTOGEAPHY. 

been shown by Mr. Maxwell Lyte tbat an alkaline jDhospliate 
or borate produces the same effect. Others again have used 
an acetate, and it will probably be found that any salt in 
which the alkaline constituent is combined with a weak acid 
will answer the purpose. In such cases, the solution of the 
chloride of gold gradually become? colorless by an interchange 
of elements, as above described. 

CONDITIONS WHICH AFFECT THE ACTION OF THE ALKALINE 
GOLD TONING BATH. 

a. Degree of Coricentration of the Bath. — The exact amount 
of dilution with water in the alkaline toning bath is not of 
much consequence. A strong bath acts quickly, but is more 
liable to destroy the middle tints than a weaker solution left 
upon the image for a longer time. The proportion of car- 
bonate of soda is also of minor importance, but it is desirable 
to use the minimum quantity, since a highly alkaline liquid 
dissolves the size of the paper, and causes a blistering of the 
albumen when the picture is laid in the fixing bath. 

b. The Length of Time after Mixing. — The two solutions of 
chloride of gold and carbonate of soda may be kept separately 
for any length of time, but on mixing them a chemical change 
ensues, and the liquid gradually loses its yellow color. When 
the solution has become nearly colorless it is in the best state 
for use, and will remain so for many hours ; but, after a 
longer keeping of days or weeks, it loses its activity, and 
deposits gold in the metallic state on the sides of the bottle ; 
alkaline solutions of chloride of gold being easily reduced by 
traces of organic matter. 

c. Presence of Tree Nitrate of Silver on the Surface of the 
Proof. — It is quite necessary to wash away the free nitrate of 
silver from the pictures before they are immersed in the 
toning bath, otherwise the bath will become turbid and dis- 
colored. Nitrate of silver added to an alkaline solution of 
chloride of gold throws down not only carbonate of silver and 
chloride of silver, but also a brown powder, consisting of per- 
oxide of gold, or metallic gold, and the supernatant liquid 
contains little else than nitrate of soda. 

d. Temperature of the Solution. — The action of the bath is 
much increased by carefully heating it to about 120 deg. Fahr., 
before the prints are immersed, and this method has also the 
advantage of bringing the solution at once to the nearly color- 



TONING BY GOLD. 205 

less condition, in wliicli it contains more oxide and less 
chloride of gold. The plan of warming the bath, however, is 
not often adopted by operators, since a similar result can gen- 
erally be obtained by continuing the action of the cold solu- 
tion for a longer time. 

. e. Presence of Iodide of Silver. — When iodide of silver is 
present in a proof, the toning action of the alkaline chloride 
appears to be less rapid than when the image is formed on 
chloride only. Hence sensitive paper is not improved, but 
rather the contrary, by associating iodide with the chloride in 
salting, as some authors have advised. It is also unwise to 
employ a nitrate bath which has been previously used for 
negatives, and contains iodide of silver dissolved. On floating 
the paper upon such a bath, the albumen/abstracts the iodide 
from it in virtue of a chemical aflinity, and the printed proof 
upon such paper remains for a comparatively long time in the 
toning solution without being colored by the gold, 

f. Mode of Prepariyig the Paper. — The preparation of the 
paper is a matter of greater nicety in the alkaline gold toning 
process than in the old mode of toning prints by sulphur, 
since the aflinity which secures the coloring action is stronger 
in the latter case than in the former. By adding an alkali to 
chloride of gold you improve the color of the print, but 
retard, the process of coloring, since, as before shown, the 
oxide of gold produced by the alkali is of itself nearly or 
quite inert, and the chlorine is the element which possesses 
the affinity for silver, and thus determines the change. 
Hence, although a defective sample of paper may tone evenly 
in a sulphur bath, in the. case of the alkaline gold bath there 
would be either spots or markings, which would remain of a 
red color and refuse to tone. 

One cause of unevenness of toning is an improper mode of 
applying the albumen to the paper ; for, albumen being a 
glairy substance, and having a strong tendency to run into 
lines, much will necessarily depend upon the manipulation. 
We might indeed remedy this defect by avoiding the use of 
albumen, but it is found that in the chloride of gold toning 
process, the proofs acquire an inky shade, unless the image is 
printed upon the maximum quantity of organic matter, so as 
to encourage redness as much as possible. A hard-sized gela- 
tinous English paper, with a minimum of chloride in the salt- 
ing bath, or a foreign starch-sized paper, salted with a mixed 
citrate and chloride, will produce an agreeable effect. Mr. 
Waterhouse, of Halifax, gives a formula in which the organic 



206 THEORY OF PHOTOGKAPHY. 

matter is caseine dissolved in highly dilute potash ; there is 
no gloss, aud the finished pictures ai-e of a purple tone. If 
albumen be used, the quality of the paper itself will be the 
principal point to attend to ; but, in addition to this, the state 
of the albumen must be noted, since sourness and decomposi- 
tion are injurious, not only to the sensitiveness of the paper, 
but also to the rapidity of toning. 

Additional experiments on the conditions of the paper af- 
fecting the tonins: action are much needed. To further their 
prosecution, the writer ventures to give his experience, as fol- 
lows : The more metallic the image, the greater the rapidity 
of toning by chloride of gold, but the more inky the color. 
Organic bodies which encourage the production of a subsalt of 
silver under the infiuence of the light, hinder the deposition 
of gold, but improve the color by imparting a shade of red. 
There are, however, differences between these organic bodies, 
ascertainable only by experiment, for although albumen and 
citrate both favor the reduction of the silver to the state of a 
subsalt, it will be found on trial that citrate papers tone with 
greater facility than albumenized papers. Mr. Mabley, the 
Honorary Secretary of the Manchester I*hotographic Society, 
states that the image toned more rapidly when the paper was 
excited on a silver bath containing a little free nitric acid ; 
this accords with theory, because, in presence of a strong acid, 
the reduction approaches more nearly to the state of metal — 
nevertheless the color so obtained is not agreeable, and a 
warmer shade will usually be produced by albumenized paper 
which, on its removal from the printing frame, exhibits an 
image of a brick-red, and not of a violet-blue, tone. 

g. Time of Keeping the Paper. — If a print be formed on al- 
bumenized paper, and kept for a long time before toning, 
there will commonly be some difficulty in securing a strong 
deposit of gold. This is probably due to a change which 
takes place in the paper, from the organic matter reacting by 
degrees upon the nitrate of silver, and it has been stated that 
the same change is more gradually produced when the sensi- 
tive papers are preserved for many months in a dry state by 
means of chloride of calcium or sulphuric acid ; such papers 
darken in the sun, but if the print be subjected to the action 
of a gold toning bath at the same time with one on paper 
newly sensitized, the latter will be found to tone with the 
greater rapidity. 



FIXING THE PROOF. 20T 

Section III. 
Fixing the Proof. 

The condition for a proper lixing of the proof are not al- 
ways undei-stood by operators, and consequently they have no 
certain guide as to how long the prints should remain in the 
fixing bath. 

The time occupied in fixing will, of course, vary with the 
strength of the sohition employed; but there are simple rules 
which may be followed. In the act of dissolving the unal- 
tered chloride of silver in the proof, the fixing solution of 
hyposulphite of soda converts it into hyposnlphite of silver 
(p. 126), which is soluble in an excess of hyposulphite of soda. 
But if there be an insufficient excess — that is, if the bath be 
too weak, or the print removed from it too speedily — then the 
hyposulphite of silver is not perfectly dissolved, and begins by 
degrees to decompose, producing a brown deposit in the tissue 
of the paper. This deposit, called measles, which has the ap- 
pearance of yellow spots and patches, is not usually seen upon 
the surface of the print, but becomes very evident when the 
print is held up to the light, or if it be split in half, which 
can readily be done by gluing it between two flat surfaces of 
deal and then forcing them asunder. 

The Reaction of Hyposulphite of Soda with Nirate of 
Silver. — In order to understand more fully how decomposi- 
tion of hyposnlphite of silver may disturb the process of fix- 
ing, the peculiar properties of this salt should be studied. 
With this view, nitrate of silver and hyposulphite of soda may 
be mixed in equivalent proportions, viz., about twenty-one 
grains of the former salt to sixteen grains of the latter, first 
dissolving each in separate vessels in half an ounce of distilled 
water. These solutions are to be added to each othel- and 
well agitated ; immediately a dense deposit forms, which is 
hyposulphite of silver. 

At this point a curious series of changes commences. The 
precipitate, at first white and curdy, soon alters in color ; it 
becomes canary -yellow, then of a rich orange-yellow, after- 
wards liver-color, and finally black. The rationale of these 
changes is explained to a certain extent by studying the com- 
position of the hyposulphite of silver. 

The formula for this substance is as follows : — 

AgO,S20,. 
But AgOjSgOg plainly equals AgS, or sulphide of silver, and 



208 THEOEY OF PHOTOGRAPHY. 

SO3, or sulphuric acid. The acid reaction assumed by the 
supernatant liquid is due therefore to sulphuric acid, and tlie 
black substance formed is sulphide of silver. The yellow and 
orange-_yellow compounds are earlier stages of the decomposi- 
tion, but their exact nature is uncertain. 

The instability of hyposulphite of silver is principally seen 
when it is in an isolated state ; the presence of an excess of 
hy23osulphite of soda renders it more permanent by forming a 
double salt, as ah'eady described at page 126. 

These facts explain the reason why, in fixing photographic 
prints, a brown deposit of snlpliide of silver sometimes forms 
in the bath and upon the picture. To obviate it, observe the 
following directions: It is especially in the reaction between 
nitrate of silver and hyposulphite of soda that the blackening 
is seen, the chloride and other insoluble salts of silver being 
dissolved, even to saturation, without any decomposition of 
the hyposulphite formed. Hence, if the print be carefully 
washed in water to remove the soluble nitrate, a compara- 
tively weak fixing bath may be employed. But if the proofs 
are taken at once from the printing frame and immersed in a 
dilute bath of hj^posulphite (one part of the salt to six or 
eight of water) without any previous washing, a shade of 
brown will be observed to pass over the surface of the print, 
and a large deposit of sulphide of silver will form as the re- 
sult of the decomposition. On the other hand, with a strong 
hyposulphite bath there will be little or no discoloration, and 
the black deposit will be absent even when the prints are im- 
mersed with the free nitrate of silver still upon the surface. 

The print must also be left for a sufficient time in the fix- 
ing bath, or some appearance of brown patches, visible only 
on looking through the paper, may occur. Each atom of 
nitrate of silver requires three atoms of hyposulphite of soda 
to form the sweet and soluble double salt, and hence, if the 
action be not continued sufficiently long, another compound 
will be formed almost insoluble, and with a greater tendency 
to decompose into sulphide of silver. Even immersion in a 
new bath of hyposulphite of soda will not hx the print when 
once decomposition of hyposulphite of silver has commenced. 
The yellow or brown compound is not entirely soluble in 
hyposulphite of soda, and consequently remains in the paper. 

Anotlier important matter ;to observe in fixing photo- 
graphic prints is the temperature of the fixing bath. When 
hyposulphite of soda is first dissolved in water, much heat is 
rendered latent, and the solution, in consequence, is almost at 



FIXING THE PROOF. ' 209 

the freezing point. In this state of things prints are fixed 
slowly and with difficulty, whereas at 70 deg. or 80 deg. Fahr., 
the action is more rapid. In the cold winter months, positives 
should be left in the bath at least twice as long as in the sum- 
mer months, to prevent the occurrence of brown patches of 
imperfect fixation in the substance of the paper. 

The nature of the sensitive surface to be cleared of its super- 
fluous silver salt must also be considered in estimating the 
strength of the bath and the time of immersion likely to be re- 
quired. Albumenized paper, from the horny nature of its sur- 
face coating, requires a longer treatment with the hyposul- 
phite of soda than plain paper requires ; and not only so, 
but it must be borne in mind that the albumen has a property 
of combining witli nitrate of silver, and forming an insoluble 
salt which is more difficult of solution than chloride of silver. 
Gelatine also combines with nitrate of silver, and therefore 
the mere washing of a gelatinized sensitive paper for a few 
minutes in cold water does not remove the whole of the free 
nitrate. It is better in all such cases to make the fixing bath 
far stronger than theory would indicate as sufficient for the 
solution of simple chloride of silver, otherwise it will be found 
that the tixing will be insufficient, and that there will be mealy 
spots in the print; although on adding recently precipitated 
chloride of silver to the bath of hyposulphite it immediately 
dissolves. 

The writer has noticed that when sensitive paper is kept for 
some time before being used for printing, yellow patches of 
imperfect fixation are very liable to occur. The nitrate of 
silver appears gradually to undergo a partial reduction by the 
organic matter, and cannot then be so easily extracted by the 
fixing bath. It has also been observed that albumenized and 
salted paper, when kept in a damp place, or exported to warm 
and damj) climates, is liable to undergo a decomposition, in 
consequence of which the fixing bath fails to extract the whole 
of the superfluous silver after printing, and the whites remain 
discolored and spotty. 

It has been recommended by some writers on photographic 
chemistry, to saturate the fixing bath with common salt, in or- 
der to convert any nitrate of silver left in the proof by imper- 
fect washing, into chloride, and thus to prevent blackening 
from decomposition of hyposiilphite of silver. An excess of 
salt undoubtedly has the efliect thus ascribed to it, but it ap- 
pears to the writer that its use is contra-indicated by the fact 
that hyposulphite of soda in presence of chloride of sodium is 



210 THEOJBY OE PHOTOGKAPHY. 

less active as a lixinoj agent, since the chloride of sodium exer- 
cises an opposing affinity and tends to keep the silver in the 
state of chloride. 

For a reason similar to that above assigned, the fixing bath 
employed for negatives should not be used for lixing positive 
prints, since the iodide of sodium present in the former tends 
to produce pellow patches on the surface of the print ; which 
patches consist of iodide of silver in union with albuminate of 
silver or other analogous substances, and these compounds are 
very difficult of removal by the action of the fixing bath. 

ON CERTAIN INJURIOUS CONDITIONS OF THE FIXING BATH. 

The object of using the hyposulphite bath is to fix the proof 
by dissolving the unaltered silver salts, without in any way 
affecting the image. But it is a fact familiar to the photo- 
graphic chemist that the hyposulphite of soda is a substance 
which very readily yields sulphur to any bodies which possess 
an affinity for that element ; and as the reduced silver com- 
pound in the print has such an affinity, there is a tendency to 
the absorption of sulphur when the proofs are immersed in 
the bath. Hence in many cases a sulphur-toning process is 
set up, and as the picture is improved by it in appearance, as- 
suming a more purple shade, it is of ten encouraged by photog- 
raphers. Experience, however, has shown that colors bright- 
ened in this way are rendered less permanent, and, therefore, 
the careful operator will avoid sulphuration as far as possible. 
Some of the conditions which facilitate a sulphuretting action 
upon the proof, and are therefore injurious, are as follows : 

a. The Addition of an Acid to the Bath. — It was at one 
time common to add a few drops of acetic acid to the fixing 
bath of hyposulpliite of soda, immediately before immersing 
the proofs. The batli then assumes an opalescent appearance 
in the course of a few minutes, and, when this milkiness is 
perceptible, the print is rendered dark in color. The chemi- 
cal changes produced in a hyposulpliite bath by addition of 
acid may be explained thus : The acid first displaces the feeble 
hyposulphurous acid from its combination with soda. Then 
the hyposulphurous acid, not being a stable substance when 
isolated, begins spontaneously to decompose, and splits up into 
sulphurous acid — remaining dissolved in the liquid and com- 
municating the characteristic odor of burning sulphur — and 
sulphur, which separates in a finely divided state and forms a 
milky deposit. In chemical symbols, 8303=803 and S. 



FIXING THE PROOF. 211 

Observe, therefore, that free acids of all kinds must be ex- 
cluded from the iixino; bath, or, if inadvertently added, the 
liquid must afterward be neutralized bj carbonate of soda or 
chalk. 

b. Decomposition of the Bathl)y Constant Use. — It has long 
been known that hyposulphite of soda undergoes a peculiar 
change in properties when much used in fixing. The solution 
M^hen first prepared leaves the image of a red tone, M'hich is 
the characteristic color of the reduced silver salt, but it soon 
acquires the property of darkening this red color by a subse- 
quent communication of sulphur. Plence a simple fixing bath 
becomes at last a toning bath, without any addition of gold. 
This change of properties has been shown by the author to be 
due to a decomposition of hyposulphite of silver, and to the 
consequent formation of a snlphuretting body analogous in its 
properties to the tetrathionates.* During the progress of the 
change sulphide of silver is thrown down, and the supernatant 
liquid becomes acid and effervesces on the addition of chalk. 
The exact composition of the acid body above mentioned as a 
product of the spontaneous decomposition of old fixing baths 
of hyposulphite is not known, but the effects it produces on 
the prints are well understood, and can be shown to be most 
injurious ; for it not only blackens the shadows of the picture 
by communicating sulplmr, but causes a loss of the middle 
tints and destroys the pure white color of the high lights. 

In order to avoid the formation of this acid substance in 
the bath, it is most important to w^asli away all the free nitrate 
of silver; lience arises one advantage of the alkaline gold ton- 
ing process over the old methods, since this process cannot be 
carried on unless the free nitrate of silver be first washed 
away. Another precaution is to keep the bath shielded from 
a very strong light, and to put it away in a cool place. Light 
and heat both favor the decomposition of hyposulphite of sil- 
ver and cause a black deposit to be .thrown down. 

A more practical means of preserving a fixing bath for a 
time in a state in which it communicates but little sulphur to 
the proof is the addition of an alkali. The author showed in 
his early papers on the tetrathionates and their reaction with 
hyposulphite of soda that alkalies decompose the unstable sul- 
phuretting principle. Hence, if the bath be treated with pot- 
ash or carbonate of soda, an alkaline sulphide is gradually 
formed, which precipitates sulphide of silver, and in the course 

* See Journal of the Photographic Society, September and October, 1854. 



212 THEORY OF PHOTOGRAPHY. 

of a few days tlie liquid returns to its original condition and 
ceases to act as a toning agent npon tlie ])roof. Here we see 
another point of excellence in the alkaline gold toning process, 
since each print carries with it small doses of carbonate of 
soda, which effectually prevent the formation of acid in the 
bath. 

Section TV. 

On the Fading of Photographic Prints. 

For many years subsequent to the discovery of the process 
of photographic printing by Mr. Fox Talbot, it was not gen- 
erally known that pictures so produced were easily suscepti- 
ble of injury from various causes, and in particular from 
traces of the fixing agent remaining in the paper. Hence, 
due care not being taken in the proper cleansing and pres- 
ervation of the proofs, the majority of them faded. 

The present section is intended to explain practically and in 
a concise manner the causes of the fading of photographic 
prints, and the precautions which should be taken to ensure 
their permanency. 

a. Im.perfect Washing. — This is, perhaps, the most import- 
ant cause of fading and the most frequent. When hyposul- 
phite of soda is allowed to remain in the paper, even in minute 
quantity, it gradually decomposes with liberation of sulphur, 
and destroys the print in the same way, and quite as 
effectually as a solution of sulphuretted hydrogen or an alkaline 
sulphide.* 

Imperfect washing may be suspected, if the photograph 
within a few months from the date of its preparation, begins 
to get darker in color; the half-tints, whicii are the first to 
show the action, afterwards |)assing into the yellow stage, 
whilst the dark shadows remain black or brown for a longer 
time. 

The proper mode of washing photographs is sometimes 
misunderstood. The length of time during which the print 
lies in the water is a point of less importance than the con- 
tinual changing of the water. When a number of positives 
are placed together in a pan, and a tap turned upon them, the 

* The reason whv the long-continued action of sulphur fades the print 
has already been indicated at p. 200. It is because the amount of real 
silver in the image is very small, and sulphide of silver in minute quan- 
tity appears pale and yellow. 



FADING OF PKIN'I'S. 213 

circulation of fluid does not necessarily extend to the bottom. 
This is proved by the addition of a little coloring matter, 
which shows that the stream flows actively above, but at the 
lower part of the vessel, and between the prints, there is a 
stationary layer of water which is of little use in washing out 
the hyposulphite. Care should therefore be taken that the 
pictures be kept as far as possible separate from each other, 
and when running water cannot be had, that they be frequently 
moved and turned over, fresh water being constantly added. 
When this is done, and especially if the size be removed from 
the paper, in the manner presently to be advised, four or five 
hours' washing will be sufficient. It is a mistake to allow the 
pictures to remain in the water for several days ; which pro- 
duces no good effect, and may tend to encourage a putrefac- 
tive fermentation, or the formation of a white deposit upon 
the image when the water contains carbonate of lime. 

Mr. Maxwell Lyte states that traces of soluble chloride left 
in the paper are injurious to the permanency of photographic 
prints ; this indicates the propriety of avoiding the use of 
water too highly charged with saline matter, and of finishing 
the washing with a bath of rain water. 

b. Atmo8phe7'io Oxidation. — Upon examining collections 
of old photographs, it is not uncommon to find prints which 
are stated to have remained unaltered for a long while after 
their first production, but which, in the course of time, have 
lost their brilliancy, and become pale and indistinct. This 
kind of fading often commences at the corners and edges of 
the paper, and works inward towards the center ; it is princi- 
pally caused by a slow process of oxidation. 

ISulphuration in the process of toning, which has before 
been shown to occur when old and decomposed solutions of 
hyposulphite of soda are employed, always facilitates sub- 
sequent oxidation and yellowness. It is indeed difficult to 
avoid slight sulphuration unless fresh solutions of hyposulphite 
are employed in fixing, but the remarks now made on the 
injury caused by sulphur refer to its full action. 

Moisture greatly hastens the yellow change due to at- 
mospheric oxidation, and hence a state of comparative dryness 
may be said to be essential to the preservation of all photo- 
graphs. In collecting evidence upon the subject, " wet " and 
"damp" are frequently alleged as having been causes of 
fading — the prints were hung against a damp wall during 
frosty weather, in a room without a fire ; or the rain had been 
allowed to penetrate the frame ! 



21J: 



THEORY OF PHOTOGRAPHY. 



Acid matters, and also other bodies which dissolve oxide of 
silver, siicli as ammonia, etc., favor oxidation when left for a 
long time in contact with the image. The materials often 
used in sizing papers, such as alum and resin, being of an acid 
nature, are ultimately injurious to the image; and the removal 
of the size, which may easily be effected by means of hot water, 
without much injury to the tint, has the additional advantage 
of carrying out the last traces of hyposulphite of soda, and 
also the germs of fungi, which, if allowed to remain, would 
vegetate and produce a destructive mouldiness. 

The fact that acids facilitate oxidization of the image sug- 
gests, also, that 23liotographic prints should not be handled too 
frequently, or touched with the lingers more than is necessary; 
the warm hand may leave behind a trace of acid * which 
would tend, in time, to produce a yellow mark. For the same 
cause, they ought not to be laid down carelessly in any place 
likely to communicate impurity. 

The employment of albumenized paper confers a positive 
advantage in protecting the image from oxidation ; this was 
pro\'ed by careful exj)eriments performed with various oxi- 
dizers. The use of albumen indeed gives the main chance of 
preventing a print, which has been toned to blackness by sul- 
phur, from gradually becoming pale and yellow by atmos- 
pheric oxidation, 

c. Improper Modes of Mounting the Proof. — This cause of 
change might have been included with the last, since the 
fading of badly mounted prints is usually due to atmospheric 
oxidization. All cements which are of an acid nature, or 
which are liable to become sour by acetous fermentation, 
should be avoided. Flour ])aste is especially injurious, and 
many cases of fading have been traced to this cause. The 
addition of bichloride of mercury, which is often made to pre- 
vent the paste from becoming mouldy, still more unfits it for 
photograpliic use. No substance appears better than gelatine, 
which does not readily decompose, and shows no tendency to 
absorb atmospheric moisture. The deliquescent nature of 
many bodies is a point which should be borne in mind in 
mounting photographs, and hence the use of a salt like car- 
bonate of j)otash, which the writer has known to be added to 
paste to prevent the formation of acid, would be unadvisable. 

d. The Effect of Imperfect Fixation. — The earlier ]3hotog- 
raphers did not always succeed in properly fixing their prints, 

* This acid reaction of perspiration is due to lactic, acetic, and formic 
acids. 



FADING of' PRINTS. 215 

and old pliotograplis are often found thickly studded with 
spots and blotches in the tissue of the paper. These prints, 
however, are not invariably faded upon the surface, and hence 
it cannot be said that imperfect fixation will certainly end in 
the total destruction of the picture. Still, a notice of the 
subject may properly be introduced in this place, and the 
attention of the reader be once more drawn to the importance 
of examining each print by holding it against the light ; 
imperfect fixation will be shown by yellow patches in the 
body of the paper. 

e. Mr. Spiller has shown that hyposulphite of soda is not a 
perfect solvent of albuminate and some other organic com- 
pounds of silver. Hence some of these must remain in the 
paper, in spite of all care taken to the contrary, and may be 
acted on by the air in a manner similar to that described in 
the next paragraph. 

f . Impurities in the Air. — A photograph carefully prepared 
may sutfer injury from deleterious matters often present in 
the atmosphere. The air of large cities, and particularly that 
emanating from sewers and drains, contains sulphuretted 
hydrogen, and hence articles of silver-plate become tarnished 
unless placed beneath glass. The injury which a print sustains 
by exposure to air contaminated with sulphuretted hydrogen, 
is less than the tarnish produced upon the bright surface of a 
silver plate ; but it is recommended as a precautionary meas- 
ure, that photographic pictures should be protected by glass 
or kept in a portfolio. 

The products of the combustion of coal-gas are probably 
more likely than the cause last named, to be a source of in- 
jury to photographs suspended without any covering. The 
sulphur compounds in gas burn into sulphurous and sulphuric 
acid, the latter of which, in combination with ammonia, produces 
the sparkling crystals often observed upon the shop windows. 

The question as to the manner in which the photographic 
image may best be protected from these extraneous causes of 
fading has been mooted, and many plans of coating prints 
with some impervious material have been devised. If the 
pictures are to be glazed or kept in a portfolio, this of itself 
will be sufficient, but in other cases it may pei'haps be useful 
to apply a layer of spirit or gutta-percha varnish. The use of 
wax, resin, and such bodies is likely, by introducing impuri- 
ties, to act injuriously rather than otherwise. 

g. DeGomposition of Pyroxyline a Source of Injury to Col- 
lodion Photographs. — Although the present section refers 



216 THEOKY OF PHOTOGRAPHY. 

more especially to pliotograpliic prints on paper, yet a few re- 
marks on collodion images may not be out of place. Collodion 
positives and negatives are usually esteemed permanent; but 
some have been exhibited wliicb, having been put away in a 
damp place, gradually became pale and indistinct. The change 
• commences at rough edges and isolated points, leaving the 
centre, as a rule, the last affected. On examination, numerous 
cracks are often visible, thus indicating that the collodion film 
has undergone decomposition, and has liberated corrosive ox- 
ides of nitrogen. Substitution compounds containing peroxide 
of nitrogen are known to be liable to spontaneous change. 
The bitter resin produced by acting upon white sugar with 
nitro-sulphuric acid, if not kept perfectly dry, will sometimes 
evolve enough gas to destroy the cork of the bottle in which, 
it is placed ; the solution of the resin has then a strong acid 
reaction, and rapidly fades an ordinary positive print. 

These facts are interesting, and indicate that collodion pic- 
tures, containing in themselves the elements of their destruc- 
tion, should be protected from moisture by a coating of 
varnish. 

Oomjxtrative Permanency. — The photographic prints do 
not necessarily fade, in the same manner as fugitive colors, by 
a simple exposure to light and air. Bottles containing photo- 
graphs suspended in damp air were placed outside the window 
of a house with a southern aspect, for nearly three months, 
but no difference whatever could be detected between positives 
so treated and others kept in total darkness. Supposing a 
case, however, which is the common one, of injurious in- 
fluences which cannot altogether be removed, it may be use- 
ful to inquire what mode of printing gives the greatest amount 
of stability. 

Positives produced by a short exposure to light, and subse- 
quent development with, gallic acid, may be expected to be 
more permanent than ordinary sun-prints ; not that there is 
any reason to suppose the chemical composition of a de- 
veloped image to i>e necessarily peculiar, but that the use of 
gallic acid enables us to increase tlie intensity of the red pic- 
ture first formed, and to add to its stability by precipitating 
fresh silver upon it. Tbis point has not always been attended 
to ; it has been recommended to remove the print from the 
developing solution whilst in the red and early stages of de- 
velopment, and to produce the dark tones subsequently by 
means of gold ; but this plan, although giving very good re- 
sults as regards color and gradation of tone, appears to lessen 



' FADING OF FEINTS. 217 

the advantage which would otherwise accrue from the adop- 
tion of negative process, and to leave the picture, as regards 
permanency, much in the condition of" an ordinary print ob- 
tained by direct action of light. 

The original Talbotype process, in which the latent image 
is formed upon iodide of silver, produces, next to collo- 
dion, the most stable image ; but the difficulty of obtaining 
bright and warm tints on iodide of silver, will stand in the 
way of its adoption. 

Great stress has been laid upon the superior permanency 
given by the use of gold in toning, but the fact appears to be 
that the gold is precipitated in so small a proportion upon the 
lighter shades of the ]3roof, that it cannot be expected to pre- 
serve them from any destructive agencies. It will be suffi- 
cient if the toning and fixing can be fully effected without 
any real injury from communication of sulphur. 

The prints which are least stable are such as have been 
toned in acid hyposulphite baths, without gold ; and the diffi- 
culty of preserving such pictures from becoming yellow in 
the half-tones is very great. All those plans of toning in 
which acetic or hydrochloric acid is mixed with hyposulphite 
of soda, and the positive immersed whilst the liquid is in a 
milky state from precipitation of sulphur, ought studiously to 
be avoided. 

Mode of Testing the Permanence of Positives. — Perman- 
ganate of potash in very dilute solutions and slightly acidified 
with sulphuric acid, is a very delicate test for hyposulphite in 
the droppings from the corner of a print ; a mere trace will 
instantly destroy the red tint of the solution. 

A dilute solution of this salt, prepared by dissolving half a 
grain, or from that to two grains, of the salt, according to its 
purity, in one gallon of distilled water, affords a convenient 
mode of testing positives as regards their power of resisting 
oxidation. The positives must be moved occasionally, as the 
first effect is to decolorize a portion of the liquid, the per- 
manganate oxidizing the size and organic tissue of the paper. 
After an immersion of twenty minutes to half an hour, vary- 
ing with the degree of dilution, the half-tones of the picture 
begin to die out, and the full shadows become darker in color. 
The bronzed portions of the print withstand the action longer, 
but at length the whole is changed to a yellow image much 
resembling in appearance the photograph faded by sulphur. 

The most available and simple plan of testing permanence 
is to enclose the pictures in a stoppered glass bottle with a 



218 THEORY OF PHOTOGKAPHY. 

small quantity of water. If they retain their half-tones after 
a course of three months of this treatment, and do not become 
mouldy, the mode of printing followed is satisfactory. 

Boiling water will also be found useful in distinguishing 
the unstable colors produced by sulphur from those following 
the judicious employment of gold ; in all cases, the image will 
at first be reddened by the hot water, but if toned without 
sulphur it will, as a rule, recover much of its dark color on 
drying. 

A solution of chromic acid may also be applied to dis- 
tinguish prints toned by sulphur from others toned by gold ; 
the presence of metallic gold protecting the shadows of the 
picture in some measure from the action of the acid. 

The solution should be prepared as follows : 

Bichromate of potash, 6 grains. 

Strong sulphuric acid, 4 minims. 

Water, 12 ounces. 

The characteristic appearance of prints which have been 
much sulphuretted, and are very liable to fade, should be 
known. A yellow color in the lights is a bad sign ; and, if 
the half-tones are at all faint and indistinct, with an aspect of 
commencing yellowness, it is almost certain that the positive 
will not last for any considerable length of time. 




CHAPTER X. 

THE PHOTOGRAPHIC DARK ROOM. 

This is the room in which the sensitizing of collodion paper, 
etc., and also the development of the image, is carried on. It 
need not be dark in a visual, but only in a chemical sense. 
We have already seen that only certain rays of light exercise 
a chemical effect on our sensitive compounds, hence it will be 
necessary to prepare them in a room so lighted as not to 
affect them. This may be done by lighting it with orange- 
colored glass, and not working too close to the window. Red 
glass would be a more effective substitute, but as the working 
in that kind of light is exceedingly painful to the eyes, the 
other kind of glass is preferable. A lamp or candle for illu- 
mination is sometimes used ; but as the rays proceeding from 
these are often of a very actinic nature, the direct light should 
be prevented from falling on the sensitive film, by interposing 
near the source of light a screen of orange-colored glass. It 
is very easy to test the " dark room " for chemical rays ; and, 
if this has been once done when the external light is very 
povsrerful, no more testing will be necessary. Sensitize a 
collodionized plate in a nitrate bath, both of which are known 
to be in good condition. Expose the plate for a few minutes 
to such light as may find access therein. Pour on a devel- 
oper, and watch if there be any reduction of silver or dark- 
ening of the film. Should reduction take place, even to 
a small extent, it is probable that some actinic light, which 
might be troublesome, does exist in the room. But it is not 
always safe to conclude that such may be the cause of darken- 
ing, because the chemicals themselves may be in fault. Test 
again by a method which, although a little more troublesome, 
will infallibly point out the source of fogging. Excite a 
collodionized plate in the nitrate bath, and immediately place 
it in a dark frame. Pull out the slide gradually within the 
dark room at intervals of, say, one minute intervening be- 
tween each. Ten intervals may be selected, which would 
represent in the maximum ten minutes' exposure to one 
minute in the minimum. A smaU portion of the plate should 
be left non-exposed. 



220 PRACTICE OF PHOTOGEAPHY. 

Apply the developer, and watch closely for the following 
effects : 

1st. If the film remains clear all over, then in all proba- 
bility the chemicals are right, and also the dark room is suffi- 
ciently protected from actinic rays. 

2d. If regular steps of gradation of darkening take place, 
then the dark room and chemicals are certainly right, pro- 
vided, as before, no deposit whatever takes place on that por- 
tion of the plate non-exposed in the slide. 

The photographic dark room should be well ventilated, 
otherwise the continued breathing of the pent-up fumes of 
ether, etc., by the operator would eventually be prejudicial to 
health. It must be provided with a water-tap and sink, or, 
where these are not available, with a large pouring jug and 
basin for the reception of washings, etc. 

No chemicals should be admitted therein which are not 
necessary for the purposes of sensitizing, developing, etc., a 
wet collodion plate. And, as the photographer is necessarily 
working in a feeble light, it is useful to have the various solu- 
tions placed not only in different positions, but also in differ- 
ent shaped bottles, so that the operator, by the sense of touch 
alone, can put his hand on the solution required. Above all 
things, ammonia must not find a place within a dark room 
intended for the wet collodion process, because the fumes 
from it very quickly destroy a nitrate bath, and in the devel- 
opment are very prejudicial. 

With these preliminary observations, we are now in a posi- 
tion to proceed with practical work. 




CHAPTEK XI. 

FORMULAS FOE NEGATIVE SOLUTIONS. 

1st. The Plain Collodion. Take of— 

Ether, sp. gr. .725, 10 fluid ounces. 

Alcohol, sp. gr. .805, 5 " " 

Pyroxyline, 120 grains. 

As before directed, shake up the pyroxyline in a bottle with 
the alcohol, then add the ether, and again shake till all the 
pyroxyline that will dissolve has passed into solution. 

2d. To Bromo-iodize the above Collodion. Take • 
Iodide of ammonium, .... 40 grains. 

" cadmium, 40 " 

Bromide of cadmium, .... 20 " 
Alcohol, sp. gr. .810 to .815, . . 5 fluid ounces. 

Shake in a clean bottle till dissolved, and add to the above 
proportion of plain collodion ; or the bromo-iodizing salts may 
be dissolved along with the pyroxyline in the stock bottle. 
But in this case the formula will be slightly altered, and stand 
thus: 

Ether, sp. gr. .725, 10 fluid ounces. 

• Alcohol, sp. gr. .805 to .815, . . 10 " " 

Pyroxyline, 120 grains. 

Iodide of ammonium, .... 40 " 

" cadmium, 40 " 

Bromide of " 20 " 

Put the bromo-iodizing salts first into the bottle, then add 
the alcohol and afterwards the pyroxyline. Shake up well 
until the salts are dissolved, complete solution being much 
favored by the rubbing action of the pyroxyline. Lastly add 
the ether, and again shake until all that is soluble of the 
pyroxyline has been taken up. This is the plan which the 
writer invariably adopts in his own practice. 

As the iodide and bromide of cadmium are exceedingly 
stable salts when exposed to the vicissitudes of temperature, 



222 PEACTICE OF PHOTOaKAPHT. 

etc., and even in ^the presence of acids (although when nsed 
alone they are apt to render collodion too viscous), they are of 
immense value in correcting the tendency which other iodides, 
etc., have to undergo decomposition when dissolved in col- 
lodion. 

Various other formulas for bromo-iodizing collodion for 
negatives have been proposed to suit different conditions of 
light, temperature, etc., but the above may be considered ex- 
cellent for a good normal collodion suitable for general pur- 
poses. If the heat of the weather and also the light are very 
intense, more bromide may be used with advantage, probably 
because bromide of silver is less liable than the iodide to be 
over-acted on by strong impulses of bright light and by high 
temperature. 

As a rule, bromo-iodized collodion is best adapted for an iron 
developer, but occasionally a pyrogallic developer is used when 
the finest possible deposit of silver is required. The latter de- 
veloper, however, acts with less energy than iron, and there- 
fore a longer exposure is required. 

Simply Iodized Collodion. — To each fluid ounce of the 
plain collodion formulated in last page, add three drams of 
either of the following solutions : 

Formula No. 1. {Potassium lodizer.) 

Iodide of potassium, 135 OTains. 

Alcohol, sp. gr. .816, 10 fluid ounces. 

Pulverize the iodide, dissolve by agitation in a bottle, and 
filter if necessary. Solution is much assisted by placing the 
bottle for a short time in warm water, say about 120 deg. 
Tahr. If the alcohol be too strong, all the iodide will not be 
dissolved. 

Formula No. 2. {Cadmium lodiser.) 

Iodide of cadmium, lYO grains. 

Alcohol, sp. gr. .810 to .816, ... 10 fluid ounces. 

By agitation this iodide will dissolve readily in the cold. 

Formula No. 3. {Mixed lodizer.) 

Iodide of ammonium, 60 grains. 

Iodide of cadmium, 90 " 

Alcohol, sp. gr. .810 to .816, ... 10 fluid ounces. 



NITKATE BATH FOR NEGATIVES. ' 223 

By agitation these iodides will also dissolve readily in the 
cold. 

Remarks on Formula No. I. — In the earlier days of pho- 
tography, this is the one which was generally used ; but as the 
collodion iodized with it is very apt after a short time to turn 
red by the liberation of free iodine, which renders the collo- 
.dion less sensitive, it is recommended to iodize only a small 
quantity of collodion at one time. 

This iodizer usually remains colorless if kept in a dark place, 
but on exposure to light a trace of iodine is liberated, tinging 
the solution yellow. On again removing the iodizer from the 
action of light, reabsorption of the liberated iodine generally 
takes place. 

Remarks on Formula No. II. — This is the most stable of 
all iodizing solutions ; and the only drawback to its general 
use is its tendency to render the collodion glutinous, and thus 
preventing a possibility of an even coating of collodion on 
large plates in warm weather. It exhibits no tendency to de- 
composition either when in alcoholic solution, or when mixed 
with plain collodion, hence, as in the next formula, it is used 
to correct the opposite tendencies of another iodide. 

Remarks 07i Formula No. Ill — 'No iodizer equals this for 
general utility. Collodion when iodized with the ammonium 
salt alone speedily decomposes, although at first the collodion 
works remarkably well. In the formula, the proportion of 
cadmium has been so arranged, from careful experiment, as to 
correct the tendency to decomposition of the other salt, even in 
the hottest weather. A light sherry color after a time is im- 
parted to the collodion, which will remain constant for many 
months, even in the presence of a slight amount of acid, or in 
bright dayliglit. 

Another variety of iodizer similar to the last may be made 
by mixing equal parts of formulas Nos. 1 and 2. 

THE NITRATE BATH FOR NEGATIVES. 

1st. For Bromo-iodized Collodion : 

Pure nitrate of silver, 35 grains. 

Distilled water, 1 ounce. 

Make in the above proportions the required quantity of solu- 
tion, and iodize it as directed at page 152. If the solution be 
neutral to test paper, that is to say, if blue litmus paper iin- 



224 ' PKACTICE OF PHOTOGKAPHY. 

mersed in it is not changed in color after, say, fifteen seconds 
Immersion, it is quite certain that the nitrate bath in this 
state will not yield negatives free from fog, unless the collo- 
dion has become quite red by the liberation of free iodine. 
But as such collodion is very insensitive, and soon disorders the 
bath, it is better not to use it. 

To rectify the solution, pour it into the glass bath or trough, 
and, supposing we have twenty ounces of solution, thoroughly 
stir up with it four drops of dilute nitric acid (equal parts strong 
acid and distilled water) and test again with litmus paper. 
After a few seconds the blue color of the paper will probably 
assume a slightly reddish hue. If so, the solution may be fit 
for work. That fact, however, can only be ascertained by a 
practical trial with a collodion known to be good. If the de- 
veloped negative is not free from a deposit of silver in the 
deepest shadows where the light has not acted, or from fog, as 
it is technically called, add two more drops of dilute acid and 
try another plate. Most likely a nice blooming negative will 
be the result. If not, try the efEect of another drop or two of 
acid. 

The operator, however, must be very careful not to add too 
much acid, which has the effect of diminishing the sensitive- 
ness of the film, and at the same time giving a very feeble 
image. For this reason the writer recommends, and in his 
own practice invariably adopts, this progressive and tentative 
method of acidulating the silver bath. 

If by inadvertence too much acid has been added, there are 
two ways of remedying the mistake. By far the best plan is 
to add to the bath by degrees some nitrate solution of the 
same strength, which has not been acidulated, practically 
testing with plates, as before, between each addition. 

The second and more troublesome plan consists in correct- 
ing an over-acid bath with oxide of silver. Proceed thus. 
Dissolve say five grains of caustic potash or soda in an ounce 
of water ; add to this a solution of nitrate of silver till no more 
precipitate falls. The precipitate is oxide of silver. Whilst 
still freshly made, throw the whole into a filter till the liquid 
has ran through. Afterwards wash the precipitate by passing 
through the same filter two or three ounces of distilled water. 
This, besides washing away impurities, will dissolve a little of 
the oxide, but the small quantity dissolved is of no conse- 
quence. I^ext pass the ove^-acid silver solution through the 
same filter, when it will be found that the acid has dissolved 
a considerable portion of the oxide, and the filtered solution 



TO PEE8EBVE NEGA.TIVE AND POSITIVE BATHS. 225 

will now, when tested with litmus paper, be found to be neu- 
tral, or perhaps slightly alkaline. Try it as before with col- 
lodionized plates, and correct more carefully this time with 
dilute nitric acid. 

2d. For iodized collodion — 

Pure nitrate of silver, ...... 30 grains. 

Distilled water, ........ 1 ounce. 

Dissolve and iodize as before. 

It will sometimes, be found that this sensitizing sokition 
works very well for iodized collodion without any addition of 
acid. In this respect much depends on the collodion. A 
sample of collodion which has become highly colored by the 
liberation of free iodine, works best when excited in a neutral 
bath ; but it is obvious that a nitrate solution cannot long re- 
main neutral if films containing free iodine are sensitized in 
it, because the iodine combines directly with the silver, lib- 
erating the nitric acid, which having no displaced base where- 
with to combine, renders the solution more and more acid by 
the sensitizing of every successive plate. Such collodions, 
when much discolored, should not be used, as they invariably 
and quickly injure the bath. To acidulate the sensitizing 
solution for a good iodized collodion the best acid to use is 
the acetic, and it should be added tentatively, as described in 
the instructions for acidulating the bath for bromo-iodized 
collodion (see page 175). 

TO PRESERVE THE NEGATIVE AND POSITIVE BATHS EOE COLLODION. 

The following observations apply both to the nitrate solu- 
tions for negative and positive processes by means of develop- 
ment : — 

By dipping successively a large number of collodionized 
plates into a bath, the silver solution becomes changed in 
various ways. 

1st. The water absorbs some of the ether and alcohol which 
still remain in the immersed film. 

2d. When the iodine, bromine, etc., of the collodion com- 
bine with the silver, a corresponding quantity of cadmium, 
potassium, or whatever other metal is combined with the 
haloid salt, is set free, and combines in its turn with the lib- 
erated nitric acid, forming nitrates of these metals, which re- 
main in solution. These secondary nitrates are not hurtful to 



226 PKACTICE OF PHOTOGRAPHY. 

the bath, further than that they weaken its power for sensi- 
tizing. 

3d. With every successive plate there is introduced into the 
silver bath a greater or less amount of organic matter. This is 
partly derived from the fingers whilst handling the plate, and 
partly from the collodion itself. Nitrate of silver has a great 
affinity for these organic substances, and consequently the bath 
is liable after a time to become loaded with dissolved 
matter which is fatal to the production of a clearly defined 
negative or positive, unless controlled by a great deal of nitric 
or acetic acid. This remedy necessitates a very much longer 
exposure, which, for portraits at least, is not admissible. 

4th. One of the most potent destroyers of a negative or 
positive bath for collodionized plates is the formation therein 
of nitro-iodide or nitro-bromide of silver. It will be necessary 
to explain this phenomenon shortly, as the facts have now been 
fully ascertained, and the causes traced to their sources. 

W hen an iodized, bromo-iodized, or bromized collodion film 
is being sensitized in the silver bath, the first effect of the 
silver is to convert the soluble iodide, etc., into the correspond- 
ing salts of silver. When that is completed the excess of 
nitrate of silver begins to dissolve out or destroy its own off- 
spring. After a time the nitrate becomes saturated, and then 
the new compound crystallizes out in fine needle crystals in 
the nitrate bath, or more generally in the collodionized film. 
Alcohol and ether in the bath much favor the deposition of 
these crystals, which are insensitive to light, and constitute one 
of the most fertile causes of pin-holes or transparent spots in 
negatives. Simply iodized collodion is less liable to this in- 
fluence than a bromo-iodized. 

If the nitrate bath is otherwise working well, the best 
remedy is to add to it say one-third of its bulk of distilled 
water, which will throw down a large proportion of the iodide 
and bromide which it has taken up. Filter, and then dissolve 
in it nitrate of silver in proportion to the quantity of water 
added. Acidulate if necessary. 

With reference to an excess of organic matter in the nitrate 
bath, the best way to test for failure in that direction is to take a 
portion of the solution and make it slightly alkaline with am- 
monia. If by exposure to strong light the liquid does not be- 
come turbid after two hours, the organic matter is not present 
in excess. This reducing power of light on solutions of nitrate 
of silver containing organic matter is taken advantage of to 
purify them. Proceed thus. Pour the impure bath into a 



TO PRESERVE NEGATIVE AND POSITIVE BATHS. 227 

transparent wide-mouthed bottle, add ammonia drop by drop 
till reddened litmus indicates, by becoming blue, that the solu- 
tion is slightly alkaline. It is better not to stopper or cork up 
the bottle, as it is important to allow any ether or alcohol 
which the solution contains to escape. Dust may be kept out 
by placing loosely over its mouth a conical fold of thin blot- 
ting paper after the manner of a candle extinguisher. This 
does not materially interfere with the escape of vapor. The 
vessel is now placed in sunlight or strong diffused daylight, in 
a place where rain does not iind access to the interior. After 
a few days' exposure most of the organic matter will be pre- 
cipitated and reduced, in combination with j)art of the silver, 
to a black powder. The time of reduction varies, according 
to the intensity of the light, and the amount of organic matter 
present, from five to thirty days. The bath should now be 
filtered, but if it is an old one fully saturated with iodide and 
bromide of silver, it is better before filtering to add to it say 
one-third of its bulk of distilled water, the eifect of which is to 
precipitate a portion of these salts in an extremely fine state 
of division, which will pass through a very porous filter. If, 
however, the weakened silver solution is well agitated and al- 
lowed to stand for a day or so, the particles will agglomerate 
and fall to the bottom. The solution should now run through 
the filter perfectly clear. 

To render the batb again fit for work, dissolve in it pure 
nitrate of silver in the proportion of thirty-five grains nitrate 
to each ounce of distilled water used in diluting. Acidulate it 
either with nitric or acetic acid, as previously directed in 
making a new bath. 

Such solutions, when the corrections have been properly 
made, work quite as well as new ones. 




CHAPTER XII. 

MANIPULATION OF THE WET COLLODION 
PROCESSES. 

CLEANING THE GLASS PLATES. 

In selecting glass for pliotography care should be taken. 
Common window glass will answer well enough for positives 
taken direct in the camera ; but for negatives it is not suffi- 
ciently flat, and is liable to break when screwed into the pres- 
sure-frame for printing. What is called " flatted crown " will 
suit for small negatives, but unfortunately it is not always flat, 
and therefore a risk is run of its breaking when large sheets 
are used. " Patent plate," although much more expensive, 
answers better than any other kind of glass, especially for large 
sizes, and as it is perfectly flat, no risk is run of its being 
broken in the printing-frame, if grit or sand is not interposed 
between the two glasses. 

Before washing the glasses each piece should be roughened 
on the edges and at the comers, by passing round them a 
sharp file, or more quickly by making two plates do duty for 
each other. Hold their edges at an angle, taking care to keep 
the arms fully extended, to prevent spiculse from flying off 
into the eyes, then successively draw the edges sharply along 
each other. This will be a suflicient protection to the hands 
from sharp edges. 

It would not be safe to rely on the glass being photographi- 
cally clean by washing it in water, and then wiping it with 
clean- cloths or wash-leathers. There are often impurities on 
the glass which these will not remove. The safest way is to 
proceed methodically, by a plan which will insure a clean 
glass, if the glass itself contains no internal contaminations, 
which by oozing out might affect the photographic chemicals. 
Make a mixture consisting of 1 part of hydrochloric acid and 
4 parts common water. Lay the j^late down on two or three 
folds of flannel laid on a flat table. With a dabber, which 
may conveniently be made of a bung or large cork, wrapped 
round with chamois leather, rub over the surface of the glass 



WET COLLODION PEOCESSES. 229 

■with considerable pressure a few drops of the hydrochloric 
mixture. Wash off the acid under a tap, finally rinsing with 
distilled or filtered rain-water, and rear up the plates to dry 
in a place free from dust ; of course taking care to mark a 
corner of the cleansed side by any device which may be 
deemed most convenient. These glasses are ready for use at 
any time in the wet collodion process, by simply brushing off 
with a broad camel's-hair brush any dust that may have 
settled on the prepared surface. 

Another method of cleaning photographic glass which has 
not been previously used is very efiicacious. 

Drop on the surface to be coated with collodion a few drops 
of old collodion, thinned with ether or alcohol, or preferably 
tincture of iodine. Instantly with a dabber, kept for that 
purpose alone, energetically work over the surface of the 
plate till the liquid has been rubbed off or has evaporated. 
Breathe on the plate, and with a fresh dabber or leather 
instantly rub off the condensed moisture. The glass is ready 
for use when, by again breathing on it, the condensed moisture 
evaporates regularly and not in patches. 

The cloths or wash-leathers used for wiping photographic 
glass should be kept expressly for that purpose. The cloths 
are best made of a material known as fine " diaper," and very 
free from flocculi and loosely-adhering fibres. Wash them 
by soaking them for a night in a tolerably strong solution of 
common washing soda. Wring them out and wash them out 
in several changes — not less than ten — of clean water ; wring 
between each change ; then hang up to dry. Wash-leathers 
are similarly prepared ; but here the operator must be careful 
not to use warm water, as that would shrivel up the skins. 
They must be washed till all the soda and " dressing " has 
been removed. It will take longer to remove the latter than 
the former; but, as the dressing is insoluble in water, it is 
easy to see when the whole of it has been washed away. 
These leathers, when dry, are generally harder and less pliant 
than they ought to be. In that case, roll them up into a ball, 
and beat them with a wooden mallet until they become soft 
and pliant. 

Silk handkerchiefs for finally polishing the dry plates are 
not recommended, because in particular states of the weather 
the glass, by being rubbed with them, becomes strongly elec- 
tric, and attracts any dust which may be floating in the imme- 
diate neighborhood. Chamois leather does not so electrically 
excite the glass so powerfully, and therefore it is to be pre- 
ferred. 



230 PEACTICE OF PHOTOGKAPHY. 

To Glean Plates that have heen Previously Used. — If, 
on developing a plate, the picture is considered a failure, it is 
best to wash off the collodion, etc., at once, and set aside to 
dry, when the glass may be considered as good as new. But 
if the collodion film has been once allowed to dry, it is better 
to soak the glass for some time in a solution of common soda 
and wash as before. 

If the glass plate has been varnished, soak it for a night in 
a very strong solution of soda, when probably the film and 
varnish, after a little rubbing, will break away in patches. 
But some sort of varnishes resist this treatment. They must, 
however, give way to the extraordinary solvent power of 
pyroxylic spirit, or wood naphtha. To apply this solvent, lay 
the plate on a flat table, pour a little of the spirit on the var- 
nish, rub with a dabber, when varnish and collodion will both 
quickly be detached from the glass. These plates should then 
be soaked in soda, and washed as before. 

Strong acids, too, have a similar effect, and act very quickly. 
The writer, who, in his practice with his students, has often 
occasion to make pyroxyline, and finds on his hands at the 
end of the year a large number of varnished negatives which 
are of no use, preserves the nitro-sulphuric acid which has 
already served its original purpose. Into this the varnished 
plates are dipped one by one, by means of a glass dipper or 
clip, or a number of them are allowed to soak in the mixture 
of acids for a short time. When placed in water, the films 
are easily rubbed off. This plan is recommended only to 
those who have plenty of waste acids which would otherwise 
be thrown down the sink without doing this final service. 

General Pemarks on Gleaning Glasses. — Much time is 
often wasted in finally cleaning glasses for the reception of the 
collodion, by the operator's frequently breathing upon them 
violently, and thus projecting particles of saliva which requires 
a great deal of rubbing for removal. Gentle breathing is all 
that is required. Again, the condition of the cleaning cloths 
or leathers for finishing should be carefully attended to. They 
should never be touched with moist hands, or hands contamin- 
ated with chemicals, and when not in use they should be stored 
away in a clean cotton bag, and in a dry place. 

TO COAT THE PLATE. 

When the salted collodion has settled for some days, decant 
off a portion into a pouring bottle for use, taking care not to 



WET COLLODION PKOCESSES. 231 

disturb the sediment. The best, and at the same time cheap- 
est, pouring vessels are the tall lipped medical bottles, procur- 
able at any druggist's. They must be fitted with the best 
corks, so as to prevent the evaporation of the ether and alcohol. 
Now take a glass plate, previously cleaned, and wipe it gently 
with a broad camel's-hair brush, in order to remove any 
particles of dust which may have subsequently collected. If 
it be a plate of moderate size, it may be held by the corners in 
a horizontal position, between the forefinger and the thumb of 
the left hand. The collodion is to be poured on steadily until 
a circular pool is f 6rmed, extending nearly to the edges of the 
glass. 




Fig. 10. 

By a slight inclination of the plate the fluid is made to flow 
towards the corner marked 1 in the above diagram, until it 
nearly touches the thumb by which the glass is held ; from 
corner 1 it is passed to corner 2, held by the forefinger ; from 
2 to three, and lastly, the excess poured back into the bottle 
from the corner marked No. 4. It is next to be held over the 
bottle for a moment, until it nearly ceases to drip, and then, by 
raising the thumb a little, the direction of the plate is changed, 
so as to give a rocking movement, which makes the diagonal 
lines coalesce, and produces a smooth surface The operation 
of coating a plate with collodion must not be done hurriedly, 
and nothing is required to insure success but steadiness of 
hand and a sufficiency of the fluid poured in the first instance 
upon the plate. 

In coating larger plates, the puematic holder, which 
fixes itself by suction, will be found the most simple and use- 
ful. 

The presence of white light in the room does no injury 
until the plate has been placed in the bath, and therefore the 
door may remain open during the operation of coating since it 
would be difficult to apply the fluid even without plenty of light. 
Draughts of air, however, must be avoided as much as pos- 
sible, since they promote too rapid evaporation, and carry 



232 PRACTICE OF PHOTOGRAPHY. 

along particles of dust which may adhere to the plate. If the 
dark room is illuminated by a yellow flame from a tallow can- 
dle or by a whiter flame surrounded with orange glass, be care- 
ful not to collodionize the plate near these, nor to have an open 
bottle of collodion near them, because as the vapors of ether 
and alcohol are very imflammable, disastrous consequences 
might ensue. 

Sensitizing the Film. — As soon as the collodion on the glass 
plate has set, as it is called, or when by touching the draining 
corner the drop of collodion does not adhere to the finger, and 
this time, according to temperature, etc., may vary from ten 
seconds to a minute, the plate is rested on the glass dipper and 
lowered into the solution by a slow and steady movement ; if 
any panse be made, a horizontal line corresponding to the sur- 
face of the liquid will be formed. Then place the cover upon 
the vertical trough and darken the room, if this has not al- 
ready been done. 

Whilst the plate remains in the bath, the operator may oc- 
cupy himself in wiping out the corners and lower edges of the 
dark slide with blotting paper ; next, in measuring a sufiiciency 
of the developing solution, and in focusing the object. Then 
rinse the fingers an instant in water, to cleanse them from any 
traces of soluble impurity and return to the bath. 

The light ought to fall upon the plate at a sharp angle 
whilst it is lifted from the bath, that the operator may see the 
greasy lines upon the surface. An immersion of from two to 
three minutes will usually be sufficient to remove them in warm 
weather ; but when the temperature falls, the time must be 
prolonged. Something will depend upon the number of times 
the plate is moved up and down, and many adopt the plan of 
leaving it in five minutes, and then taking it out without any 
movement. When the liquid flows off in a uniform sheet, the 
decomposition may be considered to be perfect. The princi- 
pal impediment in this part of the process lies in the difficulty 
with which ether and water mix together, which causes the 
collodion surface on its first immersion to appear oily and cov- 
ered with streaks. By gentle motion the ether is washed away, 
and a smooth layer obtained. 

When the nitrate bath is in perfect order no harm will re- 
sult from leaving the plate in it for a longer time; but the ex- 
perience of all practical photographers points to the fact that 
nothing is gained by a prolonged immersion, and much clear- 
ness of image may be lost. 



WET COLLODION PROCESSES. 233 

The plate is next drawn gently up through the solution, re- 
moved from the dipper and held vertically over the trough for 
a minute to drain. To further drain the plate some operators 
rest the lower edge for a short time on a few folds of blotting 
paper, and also wipe the back with the same material. These 
latter precautions are, however, not necessary, if the dark slide 
is furnished with a well or trough at the bottom into which 
the solution may run. The plate is now placed on the silver 
wires of the frame, taking care that the draining ends be towards 
the bottom. In carrying the dark frame to and from the cam- 
era, and at all times when the plate is inside, make sure that 
the slide is kept in such a position that no liquid can run back 
over the film. If this precaution be not attended to many 
sorts of defects will be visible in the picture. The frame too 
should be carried very gently, else particles of dust and abraded 
wood might be set loose and settle on the surface of the film. 
These would give rise to spots in the picture either of a trans- 
parent or opaque character, according to the nature of the im- 
purity which causes them. 

All the observations contained in this chapter apply both to 
collodion negatives and positives taken in the camera. In the 
next two chapters we must make a distinction between the 
two processes and describe them separately, because in some 
respects the treatment which is best adapted for the one will 
not suit the other. 




CHAPTEE XIII. 

THE POSITIVE COLLODION PROCESS ON GLASS OR METALLIC 
PLATES (ferrotypes). 

"We have previously detailed the various operations and 
chemicals necessary before obtaining a positive picture by de- 
velopment, viz., cleaning the glass, the collodion, the sensitiz- 
ing bath, the developers, the fixing agent, and the general ob- 
servations contained in last chapter. It now remains for us to 
describe those manipulations wherein the positive process dif- 
fers from the negative. These consist mainly in the develop- 
ment. 

In developing; a glass positive the solution of sulphate of 
iron should be flowed evenly over the film, and in some quan- 
tity, so as to wash off a portion of nitrate of silver into the sink. 
In the case of negatives it is an object to save every trace of 
nitrate, and precipitate it upon the image, in order to increase 
the density ; but with positives there is a fear of getting an 
excess of intensity, and if the collodion film be tolerably creamy, 
it will always retain more than enough of the nitrate to give 
a positive free from the green or blue marks characteristic of 
delicient reduction. White stains at the margins of the plate 
are produced in great measure by the developer coming into 
contact with an excess of silver. 

Tilt the developing fluid backwards and forwards upon the 
film for about thirty seconds, or a minute, until the darkest 
shadows begin to be visible. Supposing the film to be well 
lighted with yellow light, there will be no difficulty whatever 
in ascertaining when the image is fully out ; the developer 
must then be poured off immediately, and the plate washed 
with water, or the image will be rendered too dense by fresh 
precipitation of silver, and the middle tints in the face will be 
lost in consequence. The blacks are rarely quite pure when 
the plate has been too much developed, but show either little 
spangles of silver, or a general clouding. 

When the sulphate of iron is washed off before the proper 
development is complete, the whole image looks very thin 



THE POSITIVE COLLODION PROCESS. 235 

and weak, with a blue or greenish tint ; the details in the 
shadows may perhaps be visible, if the plate was fully ex- 
posed, but usually they are more or less defective. The diffi- 
culty, with the beginner, is to distinguish the effects produced 
by wrong exposure in the camera, from others due to faulty 
development. A little care, however, will usually enable him 
to do so. If no details appear in the dark shadows, the image 
is either under-exj)osed or under-developed, but in the latter 
case the lights would be very poor and thin, as before de- 
scribed whereas in the former they would probably be vigorous, 
especially if the operator kept the sulphate of iron for a long 
time upon the film with a hope of bringing out the shadows. 

The finished picture in another case, the reverse of the last, 
may appear altogether too white and flat, without any deep 
shadows, every portion of the plate showing more or less of a 
deposit of silver. In this instance it is either over-exposed or 
over-developed, but probably the former, and particularly so 
if the lights are not of a very good color, but appear gray and 
feeble, and if the whole image shows very fairly as a negative 
when held against the light. You may always calculate upon 
diminishing excessive density of the face and light parts by 
over-exposing in the camera, but over-development makes 
them quite opaque, so that the black varnish, when placed 
beneath, does not show through. On the other hand, a pro- 
longed exposure produces a far greater effect in clouding over 
the shadows, and giving a gray color to black drapery, than 
any amount of over-development. 

When all the chemicals are in good working order, the 
finest positives are obtained by giving a rather short exposure 
in the camera, because the lights are then of a pure white, and 
the shadows transparent. With a collodion not suitable for 
positives, the image would be too intense when developed after 
a short exposure. 

Wash the plate with water, to remove the whole of the iron, 
before putting on the cyanide, else a blue deposit will often be 
formed. The cyanide may be used over and over again until 
exhausted. 

In mounting glass positives it has become a common practice 
to cover the back of the glass with black varnish, and to mount 
the picture with the collodion side towards the eye. The 
image is necessarily reversed, but the whites are very bright, 
and the shadows sufficiently clear if the collodion be of the 
transparent kind, and if the front is also varnished with a hard- 
drying transparent varnish, the image will not so readily fade 
away when thus protected from noxious fumes. 



. CHAPTEPw XIV. 

THE NEGATIVE COLLODION PROCESSES ON GLASS. 

Section I. 

WITH IODIZED COLLODION. 

The manipulations connected with cleaning, collodionizing, 
etc., the glass are the same as those described in the previous 
chapters ; but the chemicals and the mode of using them differ 
in some respects. 

As described in the last chapter, the glass having been 
cleaned, polished, and with a broad camel's-hair brush freed 
from adhering dust, is coated with iodized collodion made by 
either of the formulas at page 222. When the film has set, it 
is introduced by means of a glass dipper into the sensitizing 
bath made and corrected according to the formula at page 174. 
Extreme care must be taken to introduce into this bath along 
with the plate or otherwise, as little as possible foreign matter, 
such as that of an organic kind, inasmuch as iodide of silver 
jper se is very much more liable to be disturbed in its ac- 
tion by such contaminations than are the mixed iodide and 
bromide. 

When the plate is fully sensitized in the nitrate solution, it 
is exposed in the camera, usually for a longer time than that 
required for a positive on glass, and the slide, with all con- 
venient speed, removed into the dark room. 

The image is developed by the following solution : 

Pyrogallic acid 3 grains. 

Distilled water 1 ounce. 

Glacial acetic acid ■ . 30 minims. 

Or citric acid 2 grains. 

In hot weather less pyrogallic acid may be dissolved, if on 
trial it is found that the developer acts too energetically. 
Again, in cold weather, the restraining citric or acetic acid may 
be diminished if the development proceeds too slowly, but 
still leaves the image clear. 



NEGATIVE COLLODION PROCESSES. 23? 

On taking the plate out of the slide there will be some ac- 
cumulation of bath solution at the lower edge, which, if the 
^lass be held horizontally for a short time, will be seen to 
work its way along the surface of the film, and the effect will 
be to produce a transparent mark on applying the pyrogallic 
;acid. To prevent this annoyance, the film may be pressed ver- 
tically for an instant on absorbent blotting paper, and the 
liquid drawn off by suction. - 

The pyrogallic acid solution having been previously meas- 
ured out (about 3 drams for a plate 5x4, one ounce for a 9x7, 
^nd 12 drams for a plate of 10x8) ; hold the glass in the hand 
in the same manner as when coating it with collodion, and 
throw the liquid on evenly. It must not be poured from 
a height onto one single spot, or the whole of the nitrate of 
silver would be displaced from that spot, leaving a transpa- 
rent mark of non-reduction. The lip of the developing glass 
should be depressed until it nearly touches the film, so as to 
apply the liquid close to the edge of the plate, or to some part 
of the image which is of minor importance. Always pour out 
the whole of the measured quantity of developer, and then 
move the plate so as to keep it waving backward and forward 
upon the film. It will be quite necessary to have the source of 
light so arranged that it falls nicely upon the collodion sur- 
face, because the operator has to tilt the glass until the devel- 
oper runs into each of tiie corners, and to keep his eye upon 
the wave as it moves backward and forward, in order to pre- 
vent it from flowing off at the edges of the plate, or trickling 
■down his sleeve. 

The freedom with which the developer flows depends much 
upon the collodion and the length of time it was held before 
•dipping, since if the film retains too much ether, it will repel 
aqueous liquids. It also depends upon the bath in a measure, 
for when this solution is newly mixed, and comparatively free 
from alcohol, it will be sometimes necessary to give the plate a 
sudden jerk, to prevent the pyrogallic acid solution from stop- 
ping short of the edge. 

Notice whether the developer remains bright and clear or 
becomes turbid before the image is fully brought out. In the 
latter case the heat is too high, or the chemicals are in fault, 
^nd the chapter on " Failures " must be consulted. 

Watch the course of the development for about thirty or 
forty seconds, and especially the behavior of those parts of the 
film which at first remain yellow, but at length begin to evolve 
iine details corresponding to the shadows. Keep the pyrogallic 



238 PKACTICE OF PHOTOGRAPHY. 

acid on tlie plate until nothing more appears in these yellow 
parts, and then pour it off into a measure, and hold the plate 
for an instant against the light, so as to look through it and 
see the appearance of the negative image. 

At this point a failure very commonly occurs from the op 
perator being too tardy in his movements, and allowing the 
developer to run into oily lines, so as to produce diagonal 
black streaks upon the film. 

Having decided that the image is sufficiently intense, pro- 
ceed to the fixing ; but if it appears too weak (and allowance 
must be made for the lowering action of the hyposulphite), 
carry the development into the second stage. To do this, pour 
away ttie discolored pyrogallic acid, and w^ash out the glass 
rapidly with water ; then measure out a fresh portion, add to 
each dram about 5 drops of a 20-grain solution of nitrate of 
silver, and apply it a second time to the film, until the desired 
intensity is obtained. 

It is not recommended to add a few drops of the nitrate bath 
to the pyrogallic acid, but to employ a weaker solution of ni- 
trate of silver not saturated with iodide. This nitrate of silver 
must also be very pure, else the developer will soon discolor, 
and the shadows become stained. 

As time ought to be economized as much as possible, observe 
the following : If the image is well out before the pyrogallic 
acid becomes turbid, it will be unnecessary to throw away the 
first portion, but any amount of strengthening may be obtained 
by pouring the same developer back into the measure, and add- 
ing the nitrate of silver. Neither is it necessary to wash out 
the developing glass with water when it contains pyrogallic 
acid simply discolored, but anything approaching turbidity will 
suggest an immediate change of solution, and a washing with 
water, both of the film and the glass. The addition of fresh 
nitrate of silver to a developer in such a state is improper : the 
silver then falls irregularly, and the chance of staining is in- 
creased. 

With regard to the quantity of nitrate of silver to be added 
to the developer, nothing positive can be stated. At least three 
times as much will be required in cold as in hot weather, and 
something will depend upon the condition of the developer it- 
self. The rapidity of discoloration or turbidity in the pyro- 
gallic acid is the proper guide to follow ; and the two liquids 
may be mixed even in equal bulks if it is found that the 
mixture remains clear for twenty or thirty seconds, so as to 
give time for pouring it over the image. As a rule, however^ 



NEGATIVE COLLODION PROCESSES, 239 

the less the quantity of silver in proportion to the pyrogallic 
acid the better. 

Convenient dropping bottles can now be had at most of the 
photographic warehouses, or a small thin-lipped phial may be 
used. 

Apj)earance of the Negative linage developed with Pyro- 
galliG Acid as a Guide to the Exposure to Light. — An un- 
der-exposed plate develops slowly. By continuing the action 
of the pyrogallic acid the high lights become very black, but 
the shadows are usually defective, nothing but the yellow iodide 
being seen on those portions of the plate. After treatment 
with the hyposulphite, the picture shows well as a positive, but 
by transmitted light all the minor details are invisible ; the 
image is black and white, without any half-tone. 

An over-exposed negative develops rapidly at first, but soon 
appears to blacken slightly at every part of the plate. After the 
fixing is completed, the image is indistinct, and very little can 
be seen by reflected light but a uniform gray surface of me- 
tallic silver. By transmitted light the plates often show a red 
or brown color, and the image is faint and flat. The half- 
shadows liaving acted so long as nearly to overtake the lights, 
there is a want of proper contrast ; hence the over-exposed 
plate is the exact converse of the under-exposed, where the 
contrast between lights and shadows is too well marked, from 
the absence of intermediate tints. 

A negative which has received the proper amount of ex- 
posure, usually possesses the following characters after fixing : 
The image is partially but not folly seen by reflected light. 
In the case of a portrait, any dark portions of drapery show, 
well as a positive, but the features of the sitter are scarcely to 
be discerned. By transmitted light the figure is bright, and 
appears to stand out from the glass ; the dark shadows are 
clear, without any misty deposit of metallic silver ; the high 
lights black almost to complete opacity. 

Collodion with strong organic reactions gives a negative 
which often shows upon the surface of the glass nearly as well 
as by transmission. And if the light be at all good, the yellow 
creamy appearance which photographers term bloom, ought to 
be seen upon the image. Its absence in the case of a simply 
iodized collodion, not containing bromide, usually implies that 
some of the chemicals are out of order. 

Fixing the Negative. — Fixing the negative, or in other 
words removing from it all the unaltered iodide, is best done in 
the case of pyrogallic developed images by a strong solution of 



240 PRACTICE OF PHOTOGRAPHY. 

hyposulphite of soda. Even a very weak solution of cyanide 
of potassium is apt to weaken the half-tones of such images on 
account of the exceedingly fine deposited particles of silver, 
whereas strong hyposulphite of soda solution, unless allowed 
for a long time to act, exercises no such baneful propensities. 
The plate may be either dipped in the solution until the yellow 
appearance has disappeared, or the solution may be poured over 
the plate. 

Washing the Negative. — This may be done in any place 
where there is abundance of water, only the washing must 
not be long delayed. The best place is under a tap running 
with a gentle stream — a violent stream would wash away the 
film altogether, and still would a gentle one if the still tender 
film is placed too far below it. Wash first the picture-side 
for a short time, and drain the superfluous water from a corner 
for an instant. Next do the same with the back of the plate, 
and allow the stream also to run over the fingers to remove 
any soluble impurity resting on them ; drain again, and for a 
minute at least let the water run on and off the picture-side. 
Finally, place the negative, slightly strutting, against a flat 
board, collodion side next the board (to prevent dust from at- 
taching itself to the still soft and tender film), and resting 
on several folds of blotting-paper, which absorbs the moisture 
as fast as it drains away, and prevents dust fi-om creeping up 
by capillary attraction. In washing plates under a stream of 
water impinging on the films for the same time, there is a way 
of doing the operation thoroughly well, and a way of doing it 
badly. Simply putting the plate under a stream of water or 
soaking in any amount of water for a few minutes will not re- 
move all the fixing solution. The plate must be dramed or 
allowed to drip by holding it perpendicularly every now and 
then for a few seconds. In this way all the fixing solution 
can be removed with little water and less trouble in a very 
short time. 

The writer has seen a pernicious practice adopted by some 
operators busily occupied in studios — viz., that of rearing upt he 
newly fixed plates inside a large tub of water, leaving them to 
wash themselves whilst they were otherwise engaged. If a 
stream of water were running through the tub this plan would 
be effective ; but if the water is not changed several times, 
such washing amounts only to a greater or less dilution of the 
fixing liquid, and consequently some portion of the fixing salt 
remains in the film, and eventually destroys the image. 



NEGATIVE COLLODION PROCESSES. 241 

Section II. 

THE NEGATIVE PROCESS ON GLASS WITH BROMO-IODIZED 
COLLODION. 

This is the process now generally adopted both for portraits 
and landscapes, inasmnch as there is less liability to failure 
from impurities of the chemicals ; and not because the process 
itself possesses greater sensitiveness to luminous action. 

The constitution of the collodion has been given at page 221, 
and of the nitrate sensitizing bath at page 223. Here, also, 
the cleaning of the plate, the coUodionizing, the sensitizing, 
etc., are the same ; but the mode of development is somewhat 
different from the two processes already described. 

The primary developing solution consists of : 

Protosulphate of iron, 15 grains. 

Glacial acetic acid, 30 minims. 

Water, 1 ounce. 

Filter if there be any turbidity. 

According to temperature this normal solution may advan- 
tageously vary in the proportion of its constituents. One con- 
stituent will alter all the rest, water being the most convenient. 

In very hot weather, or when the developer acts with un- 
controllable energy, weaken the solution by increasing the 
amount of water. Or, again, in cold weather, when chemical 
action is less powerful, decrease the proportion of water. 

The mode of applying this developer is the same as that 
given at page 236. But since iron, as a rule, acts more 
rapidly than pyrogallic acid, more manual dextrerity is re- 
quired in applying the solution so as to prevent unequal de- 
Tel opment. 

It will seldom be found that sufficient density of negative 
can be obtained by the lirst development. A secondary 
stage, or reinforcement of the image, must therefore be had re- 
course to. 

To effect proper density, the first developer is washed off — 
a slight rinse with water will be enough — and while the film is 
still moist, a fresh solution of iron is poured into a clean cup, 
into w^hich a few drops of nitrate of silver, acidulated with 
acetic acid, have been previously put. Instantly apply this 
mixture to the film and spread evenly. Tilt the plate about 
gently to keep the solution in motion ; after a few seconds 
pour the redeveloper back into the cup, and look through the 



242 PEACTICE OF PHOTOGRAPHY. 

negative held up between the eye and the orange window or 
lamp of the dark room. If sufficient density of depot^it is not 
manifested, rinse the iilm with water, and again apply in the 
same way a fresli solution. 

Mr. Wilson, of Aberdeen, who has been exceedingly success- 
ful in his photographic practice, adopts a slightly different 
method of intensifying the image. After washing off the first 
developer, he applies to the film itself a few ' drops of 
nitrate of silver, and redevelops with a fresh dose of proto- 
sulphate of iron. 

Another mode of intensifying a feeble iron-developed image 
is frequently adopted. This plan is very efficacious, and the 
only danger attending its adoption is the carrying of the rede- 
velopment too far. 

Pyrogallic acid, 1 grain. 

Citric acid, 2 grains. 

Distilled water, 1 ounce. 

When the iron-developed image has been well washed, and 
before it has had time to dry, a few drops (more or less ac- 
cording to the size of the plate) of aceto-acidulated nitrate of 
silver is stirred up with sufficient of the above pyrogallic solu- 
tion, and poured on and off, till by looking through the nega- 
tive, sufficient opacity of deposited silver has been obtained. 

It must ever be borne in mind that all these reinforcing ap- 
pliances, before the plate is fixed, must be made in the dark 
room, as the film is still sensitive to actinic light. 

The plate is now fixed in a weak solution of cyanide of 
potassium, or in a strong solution of hyposulphite of soda, and 
afterwards carefully washed, as already described, and set up 
to dry. 

Intensifying after Fixing. — It will sometimes be found 
after the finished negative has been taken out into a strong 
light and examined, that the density of the deposit is not 
sufficient. In that case, take the plate back into the dark 
room before it has had time to dry, and reintensify by the 
pyrogallic acid formula and silver. This may be done in day- 
light, but the developer decomposes more rapidly, T^o more 
perfect test than this for a thoroughly washed negative can 
be found. If the fixing solution has not been entirely elim- 
inated, brown patches will be developed in those places where- 
the least traces of hyposulphite of soda or cyanide still linger.. 
Of course, the plate must be again thoroughly washed and set. 
up to dry on blotting paper. 



NEGATIVE COLLODION PROCESSES. 243 



VARNISHING THE NEGATIVE. 

Before printing positives from the negative, it is essential 
that tlie latter should be varnished or protected from injury, 
becanse a dry collodion iilm is easily scratched, and is very ab- 
sorbent of moisture, which would render it still more tender, 
or may be detach the Iilm altogether from the glass. 

The composition of the best photographic varnishes is by 
manufacturers kept secret ; but even were all the materials 
known, so much success depends on the method of compound- 
ing them that it would not be worth the photographer's while 
to manufacture them on a small scale for his own use, as the 
most efficient kinds can be purchased cheaper and probably 
much Ijetter than he himself could make them. It is not rec- 
ommended to use those varnishes which may be applied with- 
out first warming the negative, because it has been found that 
they afford a less efficient protection to the film than some 
others which require the glass to be heated. 

Hold the picture side of the glass in front of a clear fire, 
until the back of the glass feels hot — but not unbearably hot — 
when laid on the back of the hand. Gently brush off, with a 
broad camel's-hair brush, any particles of dust which may have 
settled on the film, and at once pour on the varnish, exactly 
in the same way as is done with collodion, returning the excess 
into the stock bottle. Move the plate backward and forward 
edgeways for a few seconds, taking care to keep the draining 
end lowermost. While the last drop is still hanging at the 
lower corner wipe it off with the finger or a piece of blotting 
paper, and at once hold the plate again toward the fire, 
but too not near, because the vapor of the alcohol 
or other solvent of the varnish might ignite, and the flame 
leap to the surface of the plate and instantly destroy it, 
besides, probably, seriously burning the hand. The varnish 
should dry in a few seconds into a glassy surface. Should it 
dry dead, as it is called, or without gloss, either the varnish is 
bad or the plate has not been sufficiently warmed. Waves or 
streaks on the film show that the varnish has not been prop- 
erly applied or that the plate has been too hot. 

Varnished negatives should not be printed from till some 
hours after the varnish has been applied, because a slight 
tackiness will be apparent for some time, and this might 
cause the paper to adhere to the surface and pull off with it 
portions of the film. 



244 PRACTICE OF PHOTO&EAPHT. 



STORING OF NEGATIVES. 

The preceding instructions would be incomplete without 
some observations on the best methods of storing negatives so 
as to prevent the Ulra from cracking or peeling off by vicissi- 
tudes of temperature or moisture, or becoming scratched. 
For this purpose grooved plate boxes are generally used, the 
grooves serving to keep the plates apart. There can be little 
objection to this plan, provided the boxes are always kept in a 
dry room and at an equable temperature ; and a great advan- 
tage in its favor is that, if each box is numbered and the con- 
tents tabulated, the operator can with little trouble lay his 
hand upon any negative that may be wanted for printing. 

When operations are carried on on a large scale it is usual 
to have a room iitted all round with grooved shelves, divided 
into compartments, each compartment being marked with a 
letter of the alphabet and each groove numbered. A catalogue 
of the negatives, describing the subject, is kept in the room, 
and to each entry is attached an alphabetical letter and num- 
ber corresponding to the position of every negative. Thus it 
will be very easy to lay one's hand at any time on any partic- 
■cular negative which may be required. 

A series of cupboards running round the room, and fitted 
with grooved shelves in the same way, would be an advantage, 
because thus the negatives would be protected from dust. In 
damp and cold weather a fire or stove in the room would be 
desirable. Indeed, as a general rule, the storing place should 
be kept at as equable a temperature as possible, and free from 
dampness. 

Another plan of storing negatives, which has much to rec- 
ommend it, may be advantageously adopted by amateurs who 
only work occasionally. Pack up the negatives by the dozen, 
or any other convenient number, the plates resting on one an- 
other, and having a piece of clean blotting-paper interposed 
between each to prevent scratching of the film. Tie the pack- 
age tightly up in cartridge or brown paper, and attach to it a 
catalogue of the contents. Stow away in a dry cupboard. 




CHAPTER XV. 

POKTKAITUKE. POSITIVE AND NEGATIVE. 

The Glass-house. — We commence this chapter by remarking- 
upon the advantages, or the contrary, to be expected from the 
use of a glass-house in collodion portraiture ; adding at the same 
time a few hints on the mode of its construction. Those who 
are accustomed to work beneath glass are often at fault on try- 
ing to take a picture in the open air, and obtain under such 
circumstances a hard and unpleasing portrait, with exagger- 
ated contrast of ligiit and shade. It is, without doubt, some- 
what difficult to secure fine gradation of tone when the object 
is brilliantly illuminated, partly on account of the actinic 
power of the lenses which are employed in portraiture ; and, 
therefore, unless the chemicals are prepared purposely, the 
subdued light of a glass studio is likely to give the best effect. 

Great diversity of opinion prevails as to the best mode of 
constructing a glass-house. There are, however, some prin- 
ciples generally agreed upon which may be here stated. The 
glass-house should, when practicable, have its greatest length 
in the direction of north and south, so that the sitter may be 
placed facing the north. It should be glazed with a pure 
colorless glass, as window-glass has usually a green tint, and 
when exposed for many months to the air becomes also more 
or less yellow, and absorbs the actinic rays. The glass should 
extend nearly to the ground. This is not so necessary for 
half-length portraits as for the carte de visite form, where 
equal illumination down the feet is required. A complete 
system of black-and-white curtains must be provided both for 
the roof and sides, to throw the light on that side of the sitter 
which seems to be most desirable. The roof over the sitter 
and the sides, to the distance of about three feet from the 
background, must also be opaque, since a vertical light on the 
head is apt to solarize the hair, so as to make it appear gray, 
and will also give strong shadows underneath the eye. The end 
of the room where the camera is placed had better be dark- 
ened, to prevent diffused light from falling on the lens, and 
also to relieve the eyes of the sitter from the painful expres- 
sion often caused by too much light falling on the retinae. 



246 



PRACTICE OF PHOTOGRAPHY. 



The background may be varied according to the taste of the 
operator, but to produce a neutral tint, the color should be 
somewhat like that of common brown paper. Unless, how- 
ever, the surface is quite dead, the effect of the background 
will depend upon the amount of light which shines upon it, 
so that it will produce a different color in the open air from 
that given in the glass-house. 

Proper ventilation is of the utmost importance, the heat 
being excessive in the summer months. 

The developing room is frequently made smaller than is 
advisable, the consequence of which is, that it becomes almost 
unbearable in hot weather. It ought also to be more generally 
known that the vapor of ether, when continually inhaled, has 
an extremely depressing effect upon the nervous system. 




Fig. 11. 

It may be observed that a horizontal light destroys the 
shadows entirely, and is suitable in the case of deep-sunken 
features ; but a light falling obliquely, partly from above and 
partly from the sides, and striking more upon one side of the 
face than the other, is the best in the majority of cases. 

Some operators prefer to work in a ridge-roofed glass studio, 
regulating the light by means of a series of blinds. Others, 
again, select what is called the tunnel system, of which the 
accompanying illustration will serve to give some idea. 

The tallest end of the background, B, is square — a great 
advantage in taking groups — while all the rest of the room 
not actually wanted for light may be built of any opaque 
material, to keep out the heat. The quantity of glass surface 



POETKAITUEE. 247 

need bear very little proportion to the size of the room, as 
ten feet of glass from the sitter (or from A to C) is sufficient, 
no matter how long the room may be. If the room is suffi- 
ciently wide, there need be no side-lights at all. About three 
or four feet from the background of the under surface of 
glass, from A to B, may be blackened or built of opaque 
material, as shown in the drawing. Supposing the room to 
be twelve feet wide, the highest part of the roof. A, should be 
about ten, sloping down for about eight or ten feet from the 
sitter, or A, to the lowest part, C, which may be a flat roof, 
leaded or tiled, or other opaque material, and which need be 
no higher than convenient to walk under. The sloping part 
just over the head of the sitter, A, B, may be hinged as a flap, 
to be lifted up when the weather will admit, as the warm air 
ascends to this, the highest part. This makes an admirable 
ventilator, even in this country, in summer. The back, B, 
should be placed against a tall house, if possible, while the 
rest should be placed so as to have no obstructions. The 
interior of the room should be colored a pale green color, with- 
out any pattern of any kind, as a pattern would distract the 
eye of the sitter. 

The blinds on the top lights, E, may be made to pull down 
from A to C ; spring blinds are best, if good. The side- 
lights, D, may be managed with curtains. 

Many operators prefer to work in what is called a ridge- 
roof studio ; but while the light is more difficult to control on 
account of the many flttings of blinds which will be required 
under different conditions of illumination, on the other hand 
the ridge-roof studio possesses some advantages. 

Directions for Worhing in the Open Air. — When a glass- 
house is not at command, portraits may be taken in the open 
air. Indeed, a glass-house is only necessary as a protection 
from the wind, dust, and the inclemency of the weather. By 
selecting a proper situation, and by a judicious arrangement 
of screens, portraits with as much roundness and delicacy of 
detail can be taken out of doors as in. Mr. Shadbolt, who 
has given much attention to this subject, gives the following 
directions : — 

" If we can find a spot where an angle is formed by two 
walls of a building — the walls standing respectively north-east 
and north-west — we have in fact nearly all we require ; for by 
making a light wooden frame of about eight feet square, and 
covering it with stout unbleached calico-sheeting, we form a 
roof or canopy that can be readily fitted into the angle formed 



248 PRACTICE OF PHOTOGRAPHY. 

by the two walls, and that will give the requisite shade over 
the heads of our subjects. This square frame can also be read- 
ily raised or lowered at pleasure by a little ingenuity ; and 
further, by poising our subjects so as to look towards either 
the north-west or north-east — that is, making each wall alter- 
nately the background — either side of our subject can be the 
most strongly illuminated, and the walls may either be painted 
of a suitable color for backgrounds, or covered temporarily 
with such materials as we may select for the purpose. The 
aspect indicated is not only preferable on account of uniform- 
ity of the light, but, as a rule, it will also be one most shel- 
tered from wind in the summer time, M^hen amateurs are 
most addicted to photographic pursuits. 

" With regard to the point of holding the canopy in posi- 
tion, we have only to attach cords to each corner and gather 
them to a central point above it, so as to convert the frame 
into a kind of scale. To the point of convergence of the cords 
from the angles we attach a somewhat stouter single cord, and 
this, drawn through a small pulley, or even a ring, attached to 
the wall above will enable us readily to raise it. The cord 
can be run through a second ring or pulley, to enable us to 
draw it conveniently without interfering with the canopy it- 
self ; and the weight of the same may be counterpoised by a 
piece of iron, or even by a large stone. Lastly, a couple of 
long nails partly driven into the wall at the proper height 
may be so arranged as to prevent the frame from assuming 
other than a horizontal position. It is evident that with the 
aspect above indicated it will never happen that the sitter is 
exposed to the direct rays of the sun ; but when from varia- 
tion in the aspect this inconvenience does arise, it is so diffi- 
cult a matter to apply an appropriate screen out of doors, that 
it will generally be better to modify the arrangement alto- 
gether." 

When the desired conditions of a high building cannot be 
obtained, portraits may be taken in an open courtyard or gar- 
den by attending to the following precautions: — Always back 
up the sitter, if possible, by a dark ground, not less than seven 
feet square, to prevent diffused light from entering the lens. 
To make an effective screen, construct in the first instance a 
strong deal framework, consisting of a central portion with 
two lateral ones hinged to it. Cover the whole with oil-cloth, 
painted of a suitable color, and as free from gloss as possible. 
By bringing one of the two sides forward, you are enabled to 
throw a shadow on the face of the sitter, and by turning the 



PORTRAITURE. 249 

other side back the whole arrangement may be fixed and ren- 
dered firm. Probably a screen will be required above to cut 
off the vertical light ; and if so, the apparatus may be still 
further strengthened by constructing this part of a deal frame- 
work made to bolt down against the side which projects for- 
ward. 

When everything is complete, stand in front of the lens and 
look in through the glasses from below upwards, to see if there 
be any reflection from the sky or other bright objects. Un- 
less the lens be placed very near, indeed, to the sitter, it is al- 
most certain tliat there will be something of this kind, and if 
so, a foggy picture will result. A large funnel of card-board, 
lined with black velvet, should in such a case be carried out 
to a distance of about a foot in front of the lens, and the mouth 
of this funnel should be contracted as far as possible until it 
begins to cut off the corners of the field. This will render the 
image very clear, and will wonderfully improve the quality of 
the picture. Before exposing the plate, throw a black cloth 
over the end of the funnel, which will be found sufficient to 
exclude the light. 

It is on account of the size of the glasses in a portrait com- 
bination, and the fact of their presenting so large a reflecting 
surface, that the above precautions are required. If small 
diaphragms were used, the sitter might be arranged with the 
open sky for a background without producing fogging. 

It is beyond the scope of this manual to give instructions for 
posing and lighting the sitter, our main object being to des- 
cribe the various chemicals and apparatus, and to teach the 
student how to use them. They who desire valuable informa- 
tion on what may be called the sesthetics of photographic 
portraiture are referred to Lake Price's* valuable work, in 
which he treats of the composition of subject with great clear- 
ness and artistic ability. 

*" Manual of Photographic Manipulation." By Lake Price. J. & A. 
Churchill and Sons, New Burlington Street, London. 



CHAPTER XYL 

PHOTOGRAPHIC ENLAGEMENTS. 

Section I. 

OPTICAL PKINCIPLES INVOLVED. 

There are several processes by which enlargements are pro- 
duced, but we shall here confine our attention to those that are 
employed commercially. The special merits or advantages of 
each method will be discovered in the course of our remarks. 
As important interests are connected with this department, we 
shall treat this subject with as much thoroughness as space 
will permit. Previous, however, to entering upon a detailed 
description, we must make a remark upon the optical prin- 
ciples involved in the production of enlargements, for when 
this is well understood the mere process by which they 
are to be made becomes more easily managed. 

If any object — the figure of a man, for instance — standing at 
a certain distance in front of the camera, say twenty feet, were 
photographed in the usual way, a negative would be produced 
in the focus of the lens, and if such negative were replaced in 
the same camera, and taken into a darkened room, and ar- 
ranged so as to have a strong light placed behind, and as to be 
received on a screen after being transmitted through the nega- 
tive and lens by which it was taken, a sharp image of pre- 
cisely the size of the original figure would be seen projected 
on the screen, its distance from the camera being equal to that 
at which the figure originally was. The lens, therefore, has 
two foci — one before and the other behind. These are termed 
the " conjugate " foci. By altering in ever so slight a degree, 
one of the objects at one focus, the other will also be altered. 
Let any object at a great distance be focused sharply, such will 
be the solar focus ; but if it then be made to approach, re- 
focusing will be necessary, for it will be found that in propor- 
tion as it approaches the lens, so does the focus recede from it. 
Hence is established the fact that eacli lens has two foci, one 
situated at the image, and the other at the ground glass on 



PHOTOGRAPHIC ENLARGEMENTS. 251 

which it is focused. These two positions have a definite re- 
lation to each other, and hence they are called "conjugate 
foci." 

It is easy to discover the second conjugate focus of any lens, 
provided we know its equivalent focus, together with that of 
one conjugate. It is also easy to know both conjugates under 
all circumstances of variations of either of them from the 
lens ; and consequently it is easy to tell the precise distance 
the negative and ground glass should be respectively from the 
lens, in order to produce an enlargement on any required scale 
of amplification — the practical outcome of the knowledge of 
the former. 

Having ascertained by one or the other of the methods de- 
scribed in the appendix the equivalent focus of the lens by 
which the enlargements are to be made, the following rule 
determines the precise distance at which the negative must be 
placed from the lens on the one hand, and the distance that 
must intervene between the lens and the focussing screen on 
the other. Having decided upon the number of times the 
enlargement has to be greater than the original negative, add 
1 to that number, and multiply the sum by the focus of the 
lens. This gives one of the distances. To find the other, di- 
vide the focus of the lens by the times of enlargement re- 
quired, and add it to the focus. The sum is the length sought 
for. The relative place of object and image depends upon 
whether the model is to be enlarged or reduced, for the rule 
applies to either case. 

The advantage of a knowledge of this rule will be found 
when one is about to erect an enlarging camera for any special 
class of work, when by means of a simple calculation the pho- 
tographer can ascertain the precise length that a camera has 
to be made to suit any requirement. Calculated upon the 
rules just given is a table for enlarging and reducing, given at 
the end of this book, which will save all trouble in estimating 
the relative positions of object and image in ordinary work. 

Section II. 

SOLAE CAMERA ENLARGEMENTS. 

Enlarging by the solar camera, although extensively em- 
ployed in America, is very little used in England. The form 
of solar camera most commonly preferred is that introduced by 



^52 PEACTICE OF PHUTOGEAPHY. 

"Woodward, whicli is shown in the adjoining diagram, in which 
solar rays, r r, fall upon a mirror, A B, and are reflected 
upon the condenser, by which they are made to converge to a 
pointy, passing through in their course, first, Negative J, and 
then the objective L. They are projected forward to a focus 
on a screen in front, on which is fastened, by a pin the sensi- 
tive paper. The wood-work of the camera is represented by 
E F G H, and certain fittings by K D. The size of the conden- 
ser varies from nine to twenty inches ; the larger the lens is 
the more light it collects, and consequently the more quickly 
is the positive printed. For instance, if a condenser of nine 
inches is employed, and with it an exposure of half an hour is 
required to obtain an enlargement by direct printing on silver- 
chlorized paper, a condenser of double the area will print the 
picture in half the time. The focal length of the condenser 
should not be less than twice its diameter, nor more than 




Fig. 12. 

three times. If a small condenser is used, sufiicient light may 
not be collected to impress the image within a reasonable time. 
On the other hand, if a large condenser is used, the errors 
arising from spherical aberration become considerable. 

The objective, or lens L, may be an ordinary portrait com- 
bination ; but care must be taken, in this case, that the lens 
which faces the ground glass, as in ordinary work, now faces 
the negative to be reproduced, J. 

The negative should be movable in the direction of the 
axis of the optical system by means of rack and pinion, so as 
to admit to any amount of enlargement that may be required. 

An adjustable mirror. A, B, is so placed and regulated as to 
throw the sun's rays through the condenser, I, and the enlarg- 
ing lens, L. 

The management of the apparatus is very simple. It is 
sufficient to place the part, E, H, B, A, of the solar camera in 



SOLAR CAMERA E'NLARGEMENTS. 



253 



an opening in a darkened window, to communicate by means 
of the adjusting screws, B and D, tlie movements necessary 
for keeping the solar rays always reflected in the same direc 
tion, 1 f\ to properly adjust the negative, J, so that its 
■enlarged image is sharply formed on a screen, placed at a dis- 
tance and perpendicular to the optical axis of the apparatus ; 
and, lastly, to substitute for the screen a sheet of sensitive 
paper or other photographic surface. 

Considerable improvements have been made in the solar 
'Camera of Woodward, notably those by Dr. Monckhoven, 
who, to cure the evils arising from the aberrations of 
sphericity, whereby the various transmitted rays do not arrive 
at the same focal point, constructed his condenser on what is 
called the dialytic principle. This principle consists in inter- 
cepting the cone of rays from a single crown-glass condensing 
lens, by a concave lens by which the aberrations both of 




Fig. 13. 



sphericity and color may be, the former considerably, and the 
latter altogether, remedied. The accompanying sectional dia- 
gram of this camera will serve to show its construction. 

The large condenser. A, B, is what is called a crossed lens, 
one side being more convex than the other. The more 
■convex side is placed towards the source of light. The 
refracted rays are intercepted in their passage through the 
camera by the correcting lens, C, D, which is concave-convex, 
the concave side facing the con(^nser. This lens simply 
■corrects for spherical and chromatic aberrations. The wooden 
frame, E, F, which holds the negative to be enlarged, is 
movable by means of a milled-head screw, G, nearer or farther 
away from the lens, M, when required. 

The front of the camera to which the objective or enlarging 
lens is fixed, is also movable towards or from the frame, E, F, 
h^ means of the screw, L. 



254 PRACTICE OF PHOTOGRAPHY. 

In other respects, Dr. Monckhoven's solar apparatus i& 
similar to Woodward's, and the mode of using them identi- 
cally the same. At the same time, although both of them are 
called " solar cameras," they can be used for enlargements by 
artificial light. But, in this latter case, the enlarged image 
will have to be developed instead of being directly impressed,, 
on account of the feeble actinism of artificial, compared with 
solar, light. 

The printing by the solar camera has hitherto been usually 
effected upon plain salted paper, and the printing, toning, and 
fixing are conducted in a manner and to an extent similar to 
printing by means of the common printing frame. 

When an enlargement is to be finished in colors in an ex- 
pensive manner, it is now becoming more general to have it 
made on a more stable and permanent base than silver, hence 
carbon and platina printing find most favor for such a pur- 
pose. The details of these methods of printing will be found 
in another page. 

In all cases in which the sun's rays are employed in con- 
nection with a system of lenses for enlarging, it is necessary 
either that a heliostat be made use of by which to ensure the 
sun's rays being constantly directed in one way only, or that 
the camera itself be pointed straight at the sun, in which case 
the mirror is dispensed with. The camera must be mounted 
on a strong axis inclined to suit the latitude of the place in 
which it is erected, hence when it is pointed upwards to suit 
the height of the sun at any one particular hour or season, it 
may by one progressive motion be kept directed to the source 
of light from morning till evening. If a toothed wheel be 
affixed to the axis of rotation, and geared into the one of the 
wheels of an ordinary clock, the camera may be made to fol- 
low the direction of the sun with practical accuracy. This 
method of imparting motion applies equally to the mirror of 
a stationary enlarging camera as to the case of a camera 
mounted equatorially. 

Section III. 

THE COLLODION TRANSFER PROCESS. 

This being the process by which almost all the cheap en- 
largements are produced, it is more employed than any other. 
The camera by which this kind of enlarging is usually done is- 
not a camera at all in the usual acceptation of the term, but a 



THE COLLODION TRANSFER PROCESS. 255 

simple framework for supporting the negative, the lens, and 
the glass on wliich the enlargement is made. The camera 
proper is the apartment in which the operation is conducted. 

Pointing upwards through a window so as to be directed 
towards a part of the sky clear from any obstructions, such as 
-chimneys or trees, is the base board of the structure, consist- 
ing of two parallel rigid boards made of pine. At its upper 
end is fixed a frame having a square aperture in it, and fitted 
with the requisite appliances for holding negatives of the 
various sizes that are to be enlarged. A second frame, or, 
more correctly, a plain wooden board, holds the lens, and to 
permit of focnsing it is necessary that the mechanical ar- 
rangement be such as to permit this lens-holding frame to 
slide backwards and forwards upon the base-board. The or- 
dinary focusing may be effected by the rack and pinion of the 
lens, but tlie adjustment upon the base-board is necessary for 
rough adjustment. In routine work, such as the production 
of club pictures of a certain size, when once the general 
or rough focusing is determined, it will seldom have occasion 
to be disturbed, as the fine focusing can then be effected by 
the rack and pinion of the lens. Towards the lower end of 
the base-board — which as constructed by many looks like a 
sort of tramway, the boards being twelve inches (more or 
less) apart — is erected a solid slab of wood not less in dimen- 
sions than those of the largest picture it is intended to make 
— 14x17 inches, for example. JS'ear the bottom of this are 
two small projecting pins upon which to support the plate 
during exposure. The focusing screen consists of a plate of 
glass the dimensions of the plate upon which the transfer is 
to be made, and it is faced with white paper. 

The lens required is a small carte or quarter-plate combina- 
tion, and its outer end, or that upon which is the wood, must 
be next to the focusing screen. No diaphragm is employed. 

The negative, one of carte size, having been placed in its 
holder, and the lens adjusted, a large and sharp image will be 
seen projected on the surface of the focusing screen. By 
raising or lowering the negative, the position of the enlarged 
image may be adjusted with perfect accuracy upon the focus- 
sing screen. The distance between the lens and the screen is 
determined by the rule already given, although in most cases 
the adjustments are effected by aid of a certain kind of intui- 
tion, born of experience. The utmost care is required that no 
light whatever be admitted through the window into the room, 
but that which passes through the lens. In saying this, we 



256 PKACTICE OF PHOTOGRAPHY. 

except, of course, tlie special yellow light by which the de- 
velopment is effected. Faihires in producing a high class of 
enlargement in which the wliites are perfectly pure have fre- 
quently resulted from inattention to this requirement. 

The collodion for transfers may consist of any good negative 
collodion which has been prepared for several months — for the 
older it is, up to a certain stage, the cleaner will the enlarge- 
ment be. But a good negative collodion is unsuited for trans- 
fer work until it has been diluted by the addition or from one- 
third to an equal part of plain uniodized collodion. The ob- 
ject of this is to ensure a fine soft gradation of tints, from the 
highest light to the deepest shadow. The following is a form- 
ula by which several thousands of gallons of transfer collodion 
have been made, and which, in the hands of intelligent man- 
ipulators, yields pictures of the highest excellence. 

To twenty-live ounces of plain collodion, containing about 
seven or eight grains of pyroxyline per ounce, add a bromo- 
iodizer, composed of the following: 

Iodide of cadmium, 65 grains. 

" of ammonium, 25 " 

Bromide of cadmium, 19 " 

" of ammonium, 11 " 

Alcohol, 5 ounces. 

Provided a good sample of soluble cotton has been obtained, 
this forms a transfer collodion which fulfills every requirement. 
It is desirable to add to it so much of an alcoholic solu- 
tion of iodine as to impart a deep sherry color, although this 
is not required if the collodion be allowed to stand for a few 
months after mixing before being used. It is always desirable 
that transfer collodion be made in large quantities, because by 
keeping for a few months, or even over a year, it acquires a 
charming ripeness that cannot be imparted by the admixture 
of iodine or bromine. Absolutely bare glass in the highest 
lights is an indispensable condition in collodion transfers. 

The silver bath should not exceed twenty grains to the 
ounce. 

The exposure must be determined by experience. If the 
negative be placed so as to be backed by blue sky, the expos- 
ure will be longer than if white clouds formed its backing ; 
and if pyrogallic acid be employed as a developer, the exposure 
will have to be much longer than in the case of protosulphate 
of iron. With the former, from five minutes upwards may 
have to be given; with the latter from 30 to 60 seconds will 



THE COLLODION TRANSFER PROCESS. 257 

suffice, unless the proportion of the restraining acid be in- 
creased to an unnecessary extent. 

A good tone is obtained bv the following developer : 

Pyrogallic acid, 100 grains. 

Citric acid, .......... 60 " 

Acetic acid, 2 ounces. 

Water, 20 " 

After this is applied, a short time will elapse ere the image 
appear, after which it will rapidly gain strength. 

The mistake into which the inexperienced most usually fall, 
is to carry the development too fai', by which a deep, heavy- 
looking, smudgy picture resul,ts in the transfer, although when 
viewed as a transparency on the glass it may seem all right. 
Experience only can guide one in this matter, and fortunately 
it is an experience that may be gained in course of an hour's 
active work. 

A twelve-grain solution of protosulphate of iron would de- 
velop the picture with a far greater degree of rapidity than 
the developer already given, and with an exposure very much 
shorter, but tlie tone would be unpleasant. Citric acid ensures 
dark tones, but to enable it to exercise this inilnence upon the 
image that is being formed, it is necessary that to counteract 
its retarding power, the strength of the iron be increased to a 
considerable extent, and also that the exposure be somewhat 
prolonged. 

Cyanide of potassium must not be had recourse to for fixing, 
on account of its tendency to give a light color to the deposited 
silver. The proper fixing agent is a saturated solution of 
hyposulphite of soda. But to recover an overdone picture, 
when it is inexpedient to make a second trial, cyanide is very 
serviceable. It should be allowed to act upon the image until 
the high lights are seen to be denuded of the silver by which 
they were obscured. Should the tone have been lightened to 
too great an extent by this treatment, it may be darkened by 
the application of a weak solution of the chlorides of either 
platinum, gold, or mercury. It is worthy of remark that a 
wash of the mercury salt, when allowed to act no further than 
to blacken the image, yields a jDicture which may be considered 
as tolerably permanent. 

The image having been fixed and washed, a sheet of trans- 
fer paper, previously soaked in cold w^ater for a few minutes 
until it has a slightly slimy feeling, is laid face down upon the 
collodion picture, pressed into contact with it, and placed away 



258 PKACTICE OF PHOTOGEAPHY. 

to dry. After a few hours it may be raised at one corner and 
stripped away from tlie glass, carrying with it the collodion 
picture. The " transfer " has now an exceedingly glossy sur- 
face, and when mounted on a card by means of thin glue, is 
ready for receiving oil colors without any preparation,' should 
it be desired to finish it in that manner. If a mat surface be 
desired, the transfer should be stripj)ed from the glass before 
it is quite dry. 

The transfer paper for this process is made by placing four 
ounces of gelatine in a quart of water, allowing it to soak for 
half an hour, and then placing the vessel containing it into 
warm water to liquefy the swoolen gelatine. Four grains of 
chrome alum, previously dissolved in a little warm water, are 
now added, and incorporated with the gelatine. Good stiff 
paper, similar to a fair sample of heavy writing-paper, having 
been previously cut into sheets the required size, is floated 
sheet by sheet on the surface of the gelatine, which is kept 
warm by a water bath. Sponging over the paper with the 
gelatine answers equally as well as floating it. 

If the transfer does not strip away properly from the plate, 
it is caused by one or the other of the following: — The glass 
was dirty and not sufiiciently prepared with French chalk, or 
the coating of the transfer paper with gelatine has been too 
thin. In either case the remedy is obvious. 



Section IV. 



THE PHOTO-CRAYON PROCESS. 

This process is closely allied to that just previously des- 
cribed, but it is simpler, and the results are more beautiful and 
artistic. 

The image is produced in the same manner, but it must be 
made upon a large plate of glass, preferably the size of the 
frame, in which it is eventually to be placed. It must also be 
vignetted ; this is done by interposing a sheet of cardboard, 
having a suitable aperture between the lens and the sensitive 
plate. But, whereas, in the collodion transfer process, pro- 
vision had to be made to allow the film to leave the glass, in 
this process it is necessary to cause it to adhere. This con- 
dition is secured by sponging diluted albumen — the white of 
one egg to a quart of water, over the surface, allowing it to be- 
come dry before collodionizing. 



ENLARGING BY ARTIFICIAL LIGHT. 259 

After the image is developed and fixed, either a warm or a 
gray tone may be imparted. Bnt seeing that the picture is to 
be framed, and present the appearance of a fine lithograph or 
crayon drawing, it is better to flow over the surface a weak 
solution of chloride of gold or chloride of platinum, by which 
both tone and permanence are imparted. When dry the plate 
is varnished. 

A toned sheet of drawing-paper, or white or tinted blotting- 
paper is now placed behind the enlarged transparency, special 
care being taken that it be in close contact with the film side. 
Upon looking at the picture through the glass it presents the 
appearance of being a fine drawing upon the paper ; and this 
illusion is increased, and the pictorial merits of the picture 
enhanced, by previously producing upon the backing paper a 
few sketchy crayon-like lines, so as to surround and merge into 
the vignetting of the bust. This must be done in a free manner 
with a soft black pencil or crayon. The appearance of the 
finished picture is such that even skillful artists upon examina- 
tion of fine examples, have imagined them to be carefully 
executed hand drawings, never suspecting their photographic 
origin, nor the extreme simplicity of the way by which the 
crayon effects were produced. Success depends upon placing 
the backing paper in close mechanical contact with the film. 
If it be not so the picture will have a hazy, out-of-focus ap- 
pearance that is unpleasant. But optical contrast between the 
backing and the image must also be avoided, else, for optical 
reasons, will the brilliant character of the picture be totally 
destroyed, and a dull image lacking all beauty be produced. 

The rapidity with which photo-crayons may be executed, their 
permanence (apart from the circumstance of their being neces- 
sarily taken upon fragile material), their small cost, and their 
beauty, render this a process which, now that all patent restric- 
tions have been removed, ought to be more generally known 
and practiced than is the case at present. 

Section V. 

ENLARGING BY ARTIFICIAL LIGHT. 

A magic lantern, having condersers of from four to six 
inches diameter, furnishes the means by which enlargements 
may be produced by artificial light. 

The source of light is of primary importance ; when the ex- 
tent to which the enlargement is to be carried is very great, 



260 PKAOTICE OF PHOTOGRAPHY- 

the lime, magnesium, or electric light will be found indispens- 
able ; but if the extent of the amplification be only two dia- 
meters, then may an oil or common gas lamp be employed. 
The lime-light, however, is that form of illumination in which 
will be found the greatest advantage, especially if the oxygen 
be kept in a compressed state in a strong iron cylinder. In 
this way it is always ready for use, and the preparation for 
making even a single enlargement does not occupy more than 
a minute, while there is no deterioration of the oxygen such as 
takes place upon its being kept over two days in a rubber- 
lined bag. 

The object-glass of the lantern must be an achromatic por- 
trait by preference ; for although any form of photographic 
■objective may be made to answer, this fulftlls better than any 
other the requirements of enlarging by artificial light. It is 
desirable that the lantern has a front capable of adjust- 
ment to such an extent as to allow of the lens being drawn 
out from the negative to an extent double that of its solar focus 
if desired ; for this degree of extension is required to fulfill a 
condition sometimes demanded — viz., the reproduction of a 
picture barely larger than the original, or, in some instances, 
no larger at all. Lengthening tubes to effect the same end 
should be had when a lantern not originally constructed for 
enlarging is being used for this purpose. One such piece of 
tube, about two inches in length, and a second of four 
inches, will be found to answer every requirement, these being 
made so as to be used either singly or screwed one into the 
other. 

When the enlarging is produced upon a surface of extreme 
sensitiveness, such as gelatine emulsion, the amplication may 
be carried to a considerable extent by the common gas-light 
or by the petroleum lamp. The modern lanterns in which the 
edges of two or more flames are presented to the condensers 
enable this kind of enlarging to be effected with satisfaction. 
The flat side of a flame may be likewise presented towards the 
condenser, but in this case it will be impossible to secure the 
highest degree of definition over the field of delineation, unless 
a diaphragm, composed of a sheet of brass with a half -inch hole 
in it, be placed immediately in front of the flame so as to limit 
its available size. For a similar reason all flames that are used 
in the production of enlargements ought to be as small as pos- 
sible ; of course, the greater their intensity the better. 



carbon enlargements. 261 

Section YI. 
carbon enlargements. 

There is no process of printing which lends itself to the pro- 
duction of valuable and really high-class enlargements as that 
by means of carbon tissue. 

The process itseK is described at page 279, to which the 
reader is referred. 

In applying to it the principle of enlarging, while it may be 
done with great success by means of the solar camera, it is 
found in practice to print them by the direct process from an 
enlarged negative. 

Negatives of this kind are made in the same manner by 
which an enlarged positive is produced from a small negative, 
only instead of its being the original small negative which is 
enlarged, it is a transparency obtained from it by one or the 
other of the methods described in the chapter devoted to trans- 
parencies. The resulting enlargement, whether obtained by 
the ordinary wet collodion or other process, is the negative 
from which is printed the carbon positive. 

Those who have seen the innumerable faded enlargements 
on silver which have been produced in the solar camera, cannot 
but wish it were made compulsory on photographers to make 
all their enlargements by a permanent process such as carbon. 




CHAPTER XVII. 

THE PRACTICAL DETAILS OF PHOTOGRAPHIC 
PRINTING. 

Section I. 

PRINTING ON PLAIN PAPER. 

So FAR as the writer is aware, there are at present in the 
market only two kinds of paper well adapted for this purpose, 
viz., Saxe and Eive, either of which may be adopted. In all 
cases there is a difference in smoothness between the two sides 
of the paper, and this is easily detected by holding the surface 
of each sheet in such a manner that a strong light falls on it at 
an acute angle. On the wrong side will be seen wire gauze 
markings, or in default of such being observed, the paper may 
be wetted at the corner, when one side will appear smoother 
than the other. The corner of the right side of the paper 
should be marked with a pencil to prevent future mistakes. 
This is not necessary when using albumenized paper, the right 
side of which is easily seen. 

Salting the plain paper. Take of — 

Chloride of ammonium or sodium . . 200 grains. 

Citrate of soda 20 " 

Water 20 ounces. 

Pour the solution into a flat glass bath, and immerse or float 
each sheet for about a minute. Hang them up to dry by 
means of clips sold for the purpose, on a string stretched across 
the room. This operation need not be performed in the dark 
room. The salted paper will keep any length of time. 

Sensitizing the plain salted paper. Take of — 

Nitrate of silver, 60 grains. 

Distilled water, 1 ounce. 

Prepare a sufficient quantity of this solution, and ponr it out 
into a flat porcelain dish. Float the marked or smooth side 



FEINTING ON ALBUMENIZED PAPEK. 26^3 

of the paper on this for about a minute in hot weather and 
longer in cold. Pick up, by the corner, with a glass or wooden 
clip, and hang up in the dark room to dry spontaneously. It 
is then ready tor use, but will remain clear and untarnished for 
twenty-four hours or more if the paper be of good quality. 
For further particulars see next section. 

Another means of sensitizing is by means of ammonio- 
nitrate of silver, sixty grains to the ounce of distilled water, 
prepared thus : — Dissolve the silver in one-half of the total 
quantity of water. Then take a strong solution of ammonia 
and drop it in carefully, stirring meanwhile with a glass rod. 
A brown precipitate of oxide of silver first forms, but on the 
addition of more ammonia it is re-dissolved. When the liquid 
appears to be clearing up, add the ammonia very cautiously, so 
as not to have excess. In order still further to secure the 
absence of free ammonia, a drop or two more of solution of 
nitrate of silver should be added, until a slight turbidity is 
again produced. Lastly, add the rest of the water. 

To apply the ammonio-nitrate sensitizer, the paper should 
not be floated on it, because the solution soon gets quite 
black, and the escape of ammonia alters its character. Lay 
the sheet to be sensitized on blotting-paper, and apply the 
liquid evenly with a broad camel's-hair brush, kept for that 
purpose only, and carefully washed. Ammonio-nitrate sen- 
sitized paper requires no gold toning bath to get the print up to 
a bluish-black tone. In this respect it differs from all other 
printing sensitizers. The difSculty often is to get the print 
warm enough in tone to be pleasing. 

The mode of conducting the printing, etc., will be described 
in the next section. 

Section II. 

PKINTING on ALBUMENIZED PAPEK. 

It is hardly worth one's while, now-a-days, to prepare his 
own albumenized paper, seeing that there are diflaculties attend- 
ing its manufacture which require special care and special ap- 
pliances. Moreover, as this is an article which does not de- 
teriorate very readily, unless subjected to moisture, it can be 
kept safely in a place dry and free from dust ; or, if it has to 
be sent to tropical or far distant countries, stored in soldered 
tins. In fact, the professional albumenizer can supply this 



264 PRACTICE OF PHOTOGRAPHY. 

article, anywhere, better and cheaper than one who only works 
occasionally can do for himself. 

It is the custom at present with albumenizers to add very 
little salt to the albumen — fi'oni five to eight grains ; whereas, 
in former times, twenty or more grains were dissolved in each 
ounce of albumen. This change is advantageous in several 
respects. It requires a weaker sensitizing bath, gives less 
violent contrasts of image, the paper keeps better after sensi- 
tizing, and altogether is better suited for the modern class of 
negatives. 

In cutting up the albumenized paper into sizes suitable for 
the bath, use a paper-knife, and do not touch the surface ex- 
cept with exceedingly dry and clean hands. 

This precaution also applies more forcibly after the paper 
has been sensitized. 

To Render the Paper Sensitive. — This operation is, of 
course, conducted in the dark room ; but it is by no means 
necessary that the room should be so dark as it must be when 
preparing sensitive films for development ; inasmuch as in the 
former case printing is completed by the direct agency of 
light and not by a developer. Any darkening that would be 
hurtful to silver-chlorized paper can therefore be detected by 
the eye. An amount of light short of this visible impression 
is permissible. 

If the albumen has not been salted at a higher rate than 
eight grains to the ounce, the strength of nitrate solution, given 
at page 262, will be amply sufficient to sensitize it. Ammo- 
nio-nitrate of silver is not admissible, because ammonio dis- 
solves albumen. 

It is often difficult to avoid air bubbles when sensitizing 
albumenized paper, especially if the solution has been fre- 
quently used. The writer adopts the following plan, which, it 
is hoped, will be understood from a verbal description. The 
solution is poured into a clean glass or porcelain tray, after be- 
ing filtered, if necessary. The operator standing close to one 
end of the dish, takes a sheet of paper by two diagonal corners 
in both hands, lowers the end next to the body on the solution, 
and instantly with the hand thus set free seizes the upper cor- 
ner, and rolls, as it were, the sheet over the surface gently and 
evenly without allowing any nitrate to get over the back of the 
paper. If the sheet is laid down rapidly, air-bubbles are sure 
to show themselves. They may easily be detected by examin- 
ing the back of the paper after it has lain for about half a 
minute on the solution. There will appear a puckering up at 



PRINTING ON ALBUMENIZED PAJEB. 265 

those points, arising from the unequal expansion of the paper. 
To remove them, raise the sheet gently by one corner, and with 
a twist of clean blotting paper dipped in the solution, moisten 
the part ; draw the bubble to the edge, or break it, and again 
lay down the paper. 

If the room is warm, one minute's floating is enough ; if 
cold, give two minutes or more. There is a disadvantage at- 
tending long contact with the nitrate solution, viz , the printed 
picture will be too much in the texture of the paper instead of 
on the surface. This detracts much from its brilKancy. Be- 
sides, the pictures are more liable to fade, inasmuch as the sil- 
ver having had time to penetrate, will come into contact with 
organic or other impurities which may exit-t in the texture. It 
is very difficult, if not impossible, to remove many of these 
afterwards by fixing and washing. 

When the paper is hung up to dry there will still be con- 
siderable drainage from the lower corner. This should be col- 
lected and put into the jar for silver residues. After about 
two minutes' draining, a small piece of blotting paper attached 
to the corner will absorb the rest. It will stick there until the 
photographic paper is dry. These pieces should also be col- 
lected and preserved, because they contain an amount of silver 
well worth the trouble (see Appendix). The blotting-paper also 
serves another purpose, for as the sensitizing paper curls up in 
drying any surplus liquid at the corner will probably touch 
some part of the surface, and there give rise to a spot of extra 
sensitiveness. 

It is better not to dry the paper artificially — that is, near a 
stove or fire — imtil the nitrate from the surface has drained 
away. Even then, before printing, it should be placed for 
about a quarter of an hour in a position where it can imbibe 
the natural humidity of the atmosphere, else when placed in 
contact with the negative in the printing frame, it will expand 
and give a blurred impression. 

If the room in which the sensitized paper is drying be damp, 
the drying will proceed too slowly, and the nitrate solution 
will go on penetrating deeper and deeper, thus producing an 
effect similar to prolonged flotation on the sensitizing bath. 

The principles to be kept in view when preparing positive 
printing paper are these. The albumenized paper should be 
bone dry, and floated for a minimum of time on a nitrate solu- 
tion strong enough to eifect complete decomposition of the 
salts on the surface only of the paper. The drying should be 
uniform and rapid, in a room where there are no sulphurous 



266 PRACTICE OF PHOTOGRAPHY, 

or other silver-reducing fumes ; and afterwards the paper, if 
not required for immediate use, stored sheet over sheet in a 
close portfolio also kept in a dry place. A good quality of 
paper thus stored will remain bright and clear for at least a 
week. 

Moisture seems to be essential to the reduction of silver- 
chlorized paper. Hence various devices have been devised 
for keeping it thoroughly desiccated in boxes containing 
quick-lime or other absorbents of moisture. These con- 
trivances answer very well ; but before laying the paper on the 
negative, it will be necessary to leave the former for a short 
time in the dark room exposed to the free action of air. 

Care of the Sensitizing Solution. — The strength of the 
nitrate bath decreases rapidly by frequent use, and hence an 
addition of nitrate of silver must occasionally be made. 

The following convenient instrument (Fig. 14) called a 
*' Silver Meter," is now almost universally 
employed to ascertain the strength of silver 
solutions. It is not quite accurate, but near 
enough for practical purposes. When greater 
accuracy is required, consult the sp. gr. table 
given in the appendix. 

It consists of a cylindrical vessel holding 
some three or four ounces, with an hydrom- 
eter of the common form. The sensitizing 
bath to be tested is poured into the glass, and 
the bulb floated in it, when the number of -pig. 14. 

grains per ounce will be indicated by the 
part of the scale corresponding to the surface of the liquid. As 
these instruments are sold for a few shillings, and consequently 
are not made with much care, it is advisable to commence 
by preparing a 90-grain solution of nitrate of silver, to verify 
the correctness of the scale. Observe also that the indications 
of the hydrometer will cease to be trustworthy if the bath con- 
tains alcohol or ether ; hence an old negative bath, properly 
analyzed, may be found to yield three or four grains of nitrate 
per ounce over the quantity given by floating. 

The solution of nitrate of silver becomes after a time dis- 
colored by the albumen, but may be used for sensitizing until 
it is nearly black. The color can be removed by animal char- 
coal, but a better plan is to use the " Kaolin," or pure white 
china clay. Shake up twenty ounces of bath, with a quarter 
of an ounce of pulverized kaolin, and filter through paper. 
Kaolin often contains carbonate of lime, and effervesces with 




PRINTING FROM THE NEGATIVE. 267 

acids ; if so, it must be purified by washing in dilute 
vinegar, or the bath may become alkaline, and dissolve off the 
albumen. 

When kaolin cannot be obtained, the bath may be decolorized 
by shaking it up with recently precipitated chloride of silver 
in the curdy state. This substance has an affinity for the 
brown sub-albuminate of silver which produces the color, and 
gradually carries it down, leaving the supernatent liquid clear ; 
but it is inferior to kaolin as a decolorizer. 

Old printing nitrate baths often become somewhat alkaline 
from the reaction of the albumen ; and the proportion of silver 
falls so low that the coagulation of the albumen is imperfect, 
and a white precipitate falls away into the bath. In such a 
case, add to each ounce ten grains of ^nitrate of silver, and a 
drop or two of glacial acetic or citric acid. 

PRINTING FROM THE NEGATIVE. 

The Exposure to Light. — For this purpose reversing-frames 
are sold, which admit of being opened at the back, in order to 
examine the progress of the darkening by light, without pro- 
ducing any disturbance of position. 

The shutter at the back is removed, and the negative laid 
flat upon the glass, collodion side uppermost. A sheet of 
sensitive paper is then placed upon the negative, sensitive side 
downwards; next comes a layer of thick felt; and the whole 
is then tightly compressed by replacing and bolting down the 
shutter. The amount of pressure required is not very con- 
siderable, but if the springs of the frame become too weak after 
a time, a few pieces of millboard may be placed beneath them. 

This operation may be conducted in the dark room ; but 
unless the light is very strong, such a precaution will be need- 
less. The time of exposure to light varies much with the 
density of the negative and the power of the actinic rays, as 
influenced by the season of the year and other obvious con- 
siderations. As a general rule, the best negatives print rather 
slowly ; whereas negatives which have been under-exposed 
and under-developed print quickly. 

In the early spring or summer, when the light is powerful, 
about ten to fifteen minutes may be required ; but from three- 
quarters of an hour to an hour and a half may be allowed in 
the winter months, even in the direct rays of the sun. It is 
always easy to judge of the length of time which will be suffi- 
cient, by exposing a small slip of the sensitive paper, un- 



268 PEACTICE OF PHOTOGRAPHY. 

shielded, to the sun's rajs, and observing how long it takes to 
reach the coppery stage of reduction. Whatever that time 
may be, nearly the same will be occupied in the printing, if 
the negative be a good one. 

In a dull light the writer has seen four days spent in getting 
one impression from a negative ; but pictures 9o obtained are 
not equal to others printed by a stronger light. The organic 
matter of the size reacts more or less upon the nitrate of 
silver, and causes yellowness of the whites of the paper ; the 
toning process is also interfered with, as will presently be 
shown. 

A light of excessive brilliancy is objectionable in printing,, 
and especially so when the thermometer stands high. Com- 
plaints of unmanageable bronzing, with obliteration of details 
in the shadows, are frequent at such times, the reducing pro- 
cess being carried on with too much activity. Hence it is 
advisable, in the summer months at least, not to print by the 
direct rays of the sun. This point is further important, 
because the excessive heat of the sun's rays often cracks the 
glasses by unequal expansion, and glues the negative firmly 
down to the sensitive paper. An exception may be made in 
the case of negatives of great intensity, which are printed 
most successfully on a w^eakly sensitized paper exposed to the' 
full rays of the sun ; a feeble light not fully penetrating the 
dark parts. 

When the darkening of the paper appears to have proceeded 
to a considerable extent, the frame is taken in and the picture 
examined. This, however, may be done in the open air, with 
care and expedition. 

If the exposure to light has been correct, the print appears 
slightly darker than it is intended to remain. The toning 
bath dissolves away the lighter shades and reduces the in- 
tensity, for which allowance is made in the exposure to light. 
A little experience soon teaches the proper point ; but much 
will depend upon the state of the toning bath, and albumen- 
ized paper will require to be printed somewhat more deeply 
than plain paper. 

If, on removal from the printing-frame, a peculiar spotted 
appearance is seen, produced by unequal darkening of the 
chloride of silver, either the nitrate bath is too weak, the sheet 
removed from its surface too speedily, or the paper is of 
inferior quality. 

On the other hand, if the general aspect of the print is a 
rich chocolate-brown in the case of albumen, a dark slate-blue 



TONING OF THE PROOF. 269 

with ammonio-nitrate paper, or a reddish purple with paper 
prepared with chloride and citrate of silver, the subsequent 
parts of the process will probably proceed well. 

If, in the exposure to ordinary diffused daylight, the shad- 
ows of the proof become very decidedly coppery before the 
lights are sufficiently printed, the negative is in fault. 
Ammonio-nitrate paper highly salted is particularly liable to 
this excess of reduction, and especially so if the light is 
powerful. 

The Toning of the Proof. — No injury results from post- 
poning this part of the process for some hours, provided the 
print be kept in a dark place. But it is not advisable to leave 
the print for a day or moi'e before toning, since a chemical 
change may take place, the effect of which will be to interfere 
with the deposition of gold in the toning bath, and to destroy 
the purity of the whites. 

The prints are iirst washed in a dish of common water, by 
immersing them one by one in a feeble light, but not neces- 
sarily in the dark-room. After separating and turning them 
■over two or three times, the water is poured off into the jar 
for preserving residues (see Appendix). More water is applied 
to them in the same way, and this also, if very milky, is con- 
signed to the waste jar. A third portion of water will proba- 
bly complete the washing. The symptom of removal of nitrate 
of silver is indicated by the hard water remaining clear. They 
are now ready for the toning bath, and their immersion therein 
should not be long delayed. 

Forrmilas for a Toning Bath. 

We may premise that, as the toning agent, chloride of gold, 
is usually sold in small hermetically sealed glass tubes con- 
taining stated quantities, and as a little of this very expensive 
salt goes a great way, the best mode of adding it to the bath is 
by measure, not weight. If the tube contains, say 30 grains of 
chloride of gold, put it in the bottom of a clean and strong 
glass bottle. Break the tube by introducing a strong glass 
rod. Pour in fifteen ounces of distilled water, which will 
speedily dissolve the whole of the chloride. Each fluid ounce 
of this solution will therefore represent 2 grains of chloride of 
gold, which should tone at least 400 square inches of prints, 
or more than one full-sized sheet of Saxe or Rice paper. But 
in this respect a good deal depends on the nature of the paper, 



270 PRACTICE OF PHOTOGKAPHY. 

the strength of the sensitizing solution, tlie nature of the 
negative, and the time that the paper has been keen kept af- 
ter sensitizing. 

Formula No. 1 (for two full-sized sheets). 

Solution of chloride of gold, . . 1 ounce or more. 
Bicarbonate of soda, , . . . . 4 to 6 grains. 
Clean water, 40 ounces. 

This solution will not keep well, and should be mixed about 
an hour before use. After toning the prints, it should be thrown 
away. 

FOKMULA No. 2. 

Solution of chloride of gold, . 1 ounce or more 

Acetate or phosphate of soda, . 100 grains. 
Clean water, 40 ounces. 

This solution will not work well until several hours after 
mixing. Indeed, it may be prepared several days previously, 
and will work all the better for keeping. After the prints 
have been toned, the solution may be bottled up and kept for 
future use. No more acetate or phosphate will be required ; 
but it will be necessary to add more gold solution immedi- 
ately, or an hour or two, before use. 

Various other salts to add to the chloride of gold have been 
worked into formulas ; but either of the above leaves nothing- 
to be desired. 

The prints are taken from the last washing water one by 
one, and immersed in the toning solution in a flat bath. It is 
better not to put many in at one time. They are kept mov- 
ing about so as to prevent unequal action. After a short time 
they will assume a bluish appearance; but that is not a true 
criterion of what their ultimate color will be. If they are 
held up between the eye and the light some correct notion is 
gained ; but neither is that a trustworthy guide, because some 
papers lose more of their tint in the fixing bath than others. 
The best plan is to try one, which is supposed fully toned, in 
the hyposulphite bath, and note how it behaves. The tone 
will redden considerably in about a minute or less, but after- 
wards will regain part of tlie color which it has lost. Experi- 
ence is the only guide in these matters. Generally it may be 
stated, 23rints on plain paper tone quicker than on an albu- 
menized surface and lose less in the fixing bath. 



FIXING THE' PEOOF. 271 

The operator must be exceedingly careful not to allow a 
trace of the hyposulphite solution to touch the toning bath, 
because this would infallibly produce prints covered with 
brown patches of sulphide of silver, which cannot be removed. 
A portion of the toning in the fixing bath does no harm fur- 
ther than weakening the latter to that extent. 

Toning should be conducted in a subdued white light, be- 
cause in a yellow or monochromatic light it is impossible to 
judge of the depth of tone. The operator will notice that 
some prints from.^the same paper assume a rich color more 
rapidly than others in the bath, while in some cases the best re- 
sults, that can be obtained is a feeble slaty -blue. The tone 
depends a good deal on the quality of the negative, all other 
circumstances being the same. Vigorous and clear negatives 
give easily toned prints, and vice versa. Temperature too af- 
fects the toning very considerably ; hence it is recommended, 
in cold weather, to warm the solution, for, within certain lim- 
its, the sooner the required tint is got, the more brilliant are 
the prints. 

Fixing the Proof. 
The fixing bath is made in the following proportions : — 

Hyposulphite of soda, 4 ounces. 

Clean water, 1 pint. 

This also is poured into a dish placed at some distance apart 
from the other, in case of splashes falling into the toning 
bath. 

In conducting this double process it is well, when conven- 
ient, to have an assistant whose sole duty is to attend to the 
prints in the fixing solution, while the principal conducts the 
toning. The latter throws the print on the hy]30snlphite solu- 
tion wdiile the assistant instantly pushes it under, and so on 
until the wdiole have been immersed. But if an assistant is 
not at hand, one careful operator can do all the work. With 
one hand he moves about the prints in the toning bath, and 
as they get ready throws them one by one with that hand on to 
the fixing bath without touching it. The other hand instantly 
pushes them under. 

While in the fixing baths the prints should be moved about 
to prevent them from sticking together, and thus being irregu- 
larly acted on. In ten minutes the fixing will be complete ; 
but as probably some of the proofs have been over-printed, 



272 PEACTICE OF PHOTOGEAPHT. 

pick out those which are too lightly or improperlj^ printed, and 
plunge them into a large basin or tub of clean water, leaving 
the others in the hyposulphite until its solvent action on the im- 
age has reduced their intensity. !N^ow an unpleasant difficulty 
occurs with prints immersed for a long time in hyposulphite 
of soda. The whites may become dingy yellow, and the half- 
tones suffer whilst the deepest shadows seem unaltered. This 
arises from the instability of hyposulphites in the presence of 
acids of any kind. 

Without entering into chemical particulars, it will be suffi- 
cient to say that hyposulphites generate their own acids when 
they act on certain salts of silver, more particularly the nitrate 
and organic. The chloride and other silver haloids are simply 
dissolved in this menstruum when neutral, without any acid 
being set free from the hyposulphite. But as the |)hoto- 
graphic print is not altogether made up of pure chloride re- 
duced by light, but is partly organic in its character, it follows 
that the hyposulphite is decomposed when placed in contact 
with the latter, and sulphur or sulphide of silver deposited. 

The moral to be drawn from these facts is this : Make the 
fixing bath decidedly alkaline by adding to it a little carbon- 
ate of soda, which will counteract the pernicious influence of 
the acid, and not interfere in any way with the dissolving 
powers of the hyposulphite. 

It is a wise plan not to use a hyposulphite bath for fixing 
more than one batch of prints, for as the salt is exceedingly un- 
stable, it may after awhile contract acid from the air, not to 
mention that which itself generates by decomposition. 

Washmg the Prints after Fixivg. 

This is a tedious operation, for unless all the soluble hypo- 
sulphite is removed from the paper, fading of the photograph 
will quickly set in. Various forms of self-acting machines 
have been devised for this purpose ; but although they save a 
great deal of labor, it is doubtful whether the best of them are 
so efficient as a systematic course of procedure by handwork 
in successive changes of water. If only about a dozen prints 
have to undergo washing, it may effectually be done in an hour 
by the following method. They are first soaked in five or six 
successive changes of water, draining tli3m between each change. 
This will remove all the hyposulphite from the exterior 
of the prints but not from the interior. A print is now taken 
and laid on a sloping glass or slate slab. A soft sponge 



DKYIN& THi; PROOFS. 273 

saturated with water, is dabbed, but not rubbed with pressure 
on the upper side, the print is taken up for an instant while the 
slab is flushed -with water, it is again laid down on the other 
side, and the dabbing process repeated. When every print 
lias gone through this operation thej are hung up, or laid 
down on clean blotting-paper, to dry spontaneously, or they 
may be dried by the fire. 

There is no more eiflcient mode of washing than the above, 
and when proofs are wanted in a hurry, it is invaluable, al- 
though entailing a great deal of labor. 

Was/ling in Dishes. — When removed from the fixing bath 
the prints are thrown into a large dish containing clean water. 
The dish should be made of glass, porcelain, glazed eathenware, 
or slate. Wooden ones are sometimes used, and will answer 
very well if the pores of the wood are filled with varnish; if 
this precaution be not taken, the hyposulphite will soak into 
the pores, and continue contaminating the water for a long 
time. Zinc dishes must not be used, as the metal will soon 
destroy a moist silver photograph placed in contact with it, by 
reducing the metal it contains. 

More prints should not be thrown into the vessel than can be 
conveniently manipulated and separated within it in abund- 
ance of water. The dish filled with water is placed in the sink 
near the tap. The prints as they come from the fixing bath 
are thrown in one by one and pushed under the water. They 
are then moved about gently with the hand, for about a 
minute, so as not to tear them nor double them up. The 
proofs are now gently pressed down to the bottom with the 
outstretched palm of one hand, while the other gradually tilts 
up the vessel into a perpendicular position so as to discharge 
the water, the prints remaining on the bottom where they will 
stick for a minute to drain. More water is let in from the 
tap, taking care that it does not fall on the pictures, and the 
same process is repeated four or five times. All this can be 
done in about ten minutes. The prints are now allowed to 
soak for an hour or more, and the above process repeated three 
or four times, before setting them aside to dry. 

It is a pernicious plan, although unfortunately one too often 
adopted, to place a large quantity of newly fixed prints in a 
dish and allow them to wash themselves by means of a small 
stream of water running in at one end and out at the 
other. The prints will stick together, and no amount of soak, 
ing can remove all the hyposulphite from those in the interior 
of the batch, if this mode of washing be employed. 



274r PEACTICE OF PHOTOGRAPHY. 

The drying should not be affected, as is sometimes done, by 
hanging the prints over strings stretched across the room, 
unless these strings have been previously cleansed from 
soluble and probably injurious matter by boiling in clean 
water. The writer adopts the plan of laying each print 
separately on a large sloping board covered with two folds of 
clean white paper. Another plan is to pin each print up by 
one of the corners to a projecting shelf. This is good ; but 
unfortunately some papers tear away by their own weight, al- 
lowing the print to fall on the ground. A third, and perhaps 
the best plan, where space admits of its ado|)tion, is to lay out 
the proofs on a sheet of clean linen or white calico stretched 
across the room. 

Defects in Silver Positives on Paper. 

1. TJie Print MarMed and StreaTcy. — These defects are 
often seen before the print is toned, if so, reject the prints. 
But more often they are only visible after the toning, a. The 
paper has been badly albumen ized, the albumen having been 
allowed to drain off in streaks, h. The sensitizing solution 
sometimes flows off in the same way when the paper is hung 
up to dry, consequently the paper prints deeper where the 
current of silver has been running. It is easy to distinguish 
between these two causes of failure. In the flrst case the 
image is redder and fainter than the rest ; in the second it is 
darker and deeper. 

2. The Prints Clean on the Surface, hnt when held tip and 
Examined Opposite a Light ^ Spotty. — This disease is called 
measles, and is sure to destroy the photograph after a very 
short time. The appearance presented is a series of irregular 
small yellow patches. These consist of that deadly enemy to 
all silver prints, viz., sulphide of silver, and lie in the texture 
of the paper. Their predisposing causes are a too weak fixing 
solution, too short immersion, but above all a bad sample of 
paper. 

3. The Print has a Cold and Faded Appearance when 
Finished. — a. The nitrate of silver has not been in sufficient 
excess in the paper, 5. The negative has not sufficient con- 
trast ; for it is impracticable to get a good print from a weak 
negative, c. The print has been over-toned. 

4. Spots on the Surface. — These, if white, arise either from 
particles of dust on the negative or surface of the paper ; if 



DEFECTS IN SILVEE PROOFS. 275 

black, from holes in the negative. Other causes may also in- 
tervene, notably metallic particles in the paper. The latter 
can always be distinguished from all other spots, there is a 
small black nucleus surrounded with a circle of white. 

5. Yellowness of the High Lights. — a. The paper has been 
kept too long after sensitizing, h. The fixing bath has been 
acid, or the action of a neutral one continued too long, 

6. Intense hronzing of the deep shadows usually arises from 
having a large excess of silver in the sensitizing bath, and 
printing from an intense negative which requires a long expo- 
sure to light. 

7. Yellow spots on the surface or hack of a ])rint arise from 
the paper, before or after sensitizing, having come in contact 
with hyposulphite of soda ; the least trace of the salt is sure to 
cause the mischief. If it has arisen from handling the paper 
with imperfectly washed hands, the impression of the fingers 
will be distinctly marked. Too great care cannot be exercised 
in handhng or laying down positive paper. Everything that 
touches it must be dry and clean. 

8. Mealiness of the jprints is a peculiar disease, which may 
arise from several causes. The appearance which goes under 
this name is represented sometimes by exceedingly small red 
spots, and at other times by similar white ones. Of course, 
opaque particles of dust, either on the negative or on the sur- 
face of the sensitive paper will, to some extent, account for 
these ; but this is not always the cause. The red specks most 
probably arise from the albumen. The writer has never seen 
a specimen of these on plain salted paper. 

9. The Print Refuses to Tone. — a. Often the fault of the 
paper, h. Long keeping of the print before toning. c. The 
toning bath has been kept too long and become inert. Add a 
little more solution of chloride of gold. 

The above constitute the most important of the failures in 
photographic printing on paper by direct contact. Various 
others exist, but the causes of them are so very obvious, that 
anyone can readily see their origin, and apply the proper 
remedy, after he has acquired a little experience. 



276 PKACTICE OF PHOTOGRAPHY. 



Seciion III. 



PLATINOTYPE PRINTINO. 

Whatever opinions may be entertained respecting tlie ten- 
dencies of silver prints to fade, it seems to be universally con- 
ceded that platinnm pictures are durable. Not only are they 
permanent, but there is a peculiar charm about them which 
renders this style of printing exceptionally valuable for a 
large class of work. Those who have to expend much money 
in having photographs colored may select the platinotype, in 
the full confidence that labor will not be thrown away, as is 
almost invariably the case when pigments are applied to silver 
prints. 

Tlie process about to be described was discovered by Mr. 
William Willis, Jr., who, starting at first by assuming, cor- 
rectly, that " platina black " was one of the most stable pig- 
ments known, next sought to discover by what means photo- 
graphs could be printed in this substance. 

Seeing that a solution of ferrous oxalate (oxalate of iron) in 
potassic oxalate (neutral oxalate of potash) was a perfect 
reducing agent of platinum, he conceived the idea that as 
ferrous oxalate can be produced by the action of light on 
ferric oxalate, it ought to follow that if paper which has 
received a wash of chloride of platinum and ferric oxalate be 
exposed under a negative and then be subjected to a bath of 
oxalate of potash, the platinum will undergo reduction in pro- 
portion to the action of the light. When removed from the 
printing frame, the pictures are feebly visible, owing to the 
reduction of the iron salt ; for, as yet, the platinum salt, 
although in contact with the iron, has taken no part in the 
matter. The light has reduced the ferric into the ferrous salt, 
and the latter is the reducer of the platinum. But as no 
chemical action can take place between two dry bodies of this 
nature, it is requisite that one of them be in solution. If the 
picture, as yet only very faintly visible, be immersed in a solu- 
tion of potassic oxalate, the ferrous oxalate formed by the 
action of the light is immediately dissolved and exerts its 
reducing action upon the platinum salt lying in contiguity, 
which becomes of a rich black color. It only remains to 
remove the remainder of the ferric oxalate by which the 
paper was sensitized, which is effected by a wash of oxalic 
acid, and the picture is finished. 



PLATINOTYPE PRINTING. . 277 

Having given the foregoing outline of this interesting pro- 
cess, we are now in a position to enter a little deeper into 
details. 

The paper is sensitized by spreading over it a mixture of 
potassic platinous chloride and ferric oxalate, made by dis- 
solving 60 grains of the dry j)latinum salt in 1 ounce of the 
iron solution, or any smaller or larger quantity in the same 
proportions. This should be used as soon as mixed, as it 
keeps in good condition for a shoi't time only, or from 10 to 
20 minutes, according to the heat of the weather. 

Suppose it be wished to sensitize a surface of paper measur- 
ing 8x10 inches, the simplest method is to place a piece of 
paper of sufficient size, with its prepared surface uppermost, 
upon an 8x10 inch glass plate, and then to fold the edges of 
the paper underneath the plate. By placing the plate upon a 
table (or, better, on a glass plate of larger size), the edges of the 
paper will be securely held between the plate and the table, 
and a smooth surface will be secured. The paper must be 
larger than the plate, to allow its edges to be turned over. 
Another method of securing a smooth surface is to place the 
paper on a glass plate of the same dimensions as the paper, 
and then to clip together the corners of the plate and the 
paper by means of American clips. Yet another method, 
which frequently answers well, is to pin the paper by its cor- 
ners to the smooth surface of a deal board. By the last two 
methods the corners of the paper are lost, which is not the 
case with the first method. 

The sensitizer is now applied to the surface by means of a 
pad of cotton wool, or, better, by a pad made by enclosing a 
tuft of cotton wool in a small piece of flannel or old gauze un- 
derclothing. 

To coat a surface measuring 8x10 inches, from 25 to 30 
minims of sensitizer will be required. This quantity should 
be measured and then poured on the middle of the sheet of 
paper, and immediately spread over the surface with a circu- 
lar motion, in as even a manner as possible, by means of the 
above described pad. The rubbing should be very gentle, 
and should be continued until the coating becomes as uniform 
as possible. 

The paper is then dried, and every means taken to keep it 
so. To this end it is stored in tubes containing a prepara- 
tion of chloride of calcium and asbestos. 

The printing is effected in exactly the same manner as or- 
dinary albumenized paper, except that over the back of the 



278 PEACTICE OF PHOTOGRAPHY. 

paper a piece of thin vulcanized rubber mnst be placed. It 
is preferable, tliougli not essential, that this sheet of rubber 
be somewhat larger than the negative, because then the edges 
of the sensitized paper are better protected from the action of 
the atmosphere. Probably in a dry climate, such as that of 
America, this and other precautions against damp would have 
less value than in England. 

An important advantage in this process is the follow- 
ing: That although it is One entailing the operation of a 
so-called " development " — in truth, a substitution process — 
yet the undeveloped image is visible, and this to such an ex- 
tent that not only is the proper time of exposure estimated 
by its appearance, but the important operations of "dodging" 
and "printing-in" are also easily carried out. 

In practice the action of the light is on the ferric oxalate 
only, and the visible image is composed of ferrous oxalate, 
carbonic dioxide being given off, thus : 

C6Fe20i2=2C2FeO,+2C02. 

Sometimes a blackening, or darkening, of the more solar- 
ized portions occurs. This is due to the presence of moisture 
in the air, which has found its way to the paper. On devel- 
opment no evil effect may be observable, but the darkening 
should be guarded against, as its presence shows that there 
has been " sailing rather too close to the wind." A darken- 
ing of the edges of the paper, where it covers that part of .the 
negative corresponding to the rebate of the dark slide, may 
often be seen, especialy when the rubber sheet does not lap 
over the edges of the negative. 

As soon as the exposure of each print is complete, the print 
should be placed in a tin can or other suitable receptacle, con- 
taining a little dry chloride of calcium, to preserve it from 
moisture until it is developed, care being taken to avoid all 
possibility of contact between the paper and the chloride, 
which would produce white or yellowish-white spots on the 
print. 

Development may be either proceeded with immediately 
after exposure, or, more conveniently, at the end of the day's 
printing. It must be conducted in a feeble white light. 

The developer consists of 130 grains of oxalate of potash in 
each ounce of water, and as it keeps for an indefinite period, 
a large quantity of the solution may be made up. 

The development is effected by floating the printed surface 
of the paper, for a few seconds, on this developing solution, 



plaiinotype' printing. 279 

wliicli is conveniently contained in a flat-bottomed disli of en- 
amelled iron or of porcelain, supported on an iron tripod. A 
Bunsen lamp, with rose-burner to spread the flame, forms the 
best means for supplying the heat ; or a spirit lamp may be 
used. If a porcelain dish be used, care should be taken to 
prevent the flame from impinging directly upon it. A tem- 
perature varying between 170 and 180 degs. Fahr., ma}'' be 
considered the standard temperature for the developer. The 
bottom of the developing dish should be covered with the de- 
veloping solution to the depth of at least one-fourth of an 
inch. 

After the batch of prints has been developed, the solution 
should be put into a bottle for future use. Before again using 
this solution for developing, it should be decanted from any 
green crystals (of no value) which may have formed, and then 
enough oxalate of potash solution should be added to it to 
bring it up to its original bulk. 

Large prints may be developed without any difiiculty on a 
narrow dish or trough (having a length equal to the breadth of 
the print), by being slowly pulled over the solution contained 
therein. 

In the foregoing operations various degrees of heat may be 
used, though a temperature at least as high as 170 deg. 
Fahr. is generally advisable, and the exposures should be made 
to suit a hot development, as a rule. But when a print has 
been over-exposed, the only possible way to remedy it is to 
develop it on a cooler bath. Now, on a cool bath the devel- 
oping action is not so energetic as it is on a hot one, and the 
detail in|the lighter portions is not so readily made visible. But 
more, the shadows also of the picture only slowly gain their 
full strength and richness ; hence it may be laid down as a rule, 
that at whatever temperature development be conducted, the 
whole of the possible action must be permitted to take place, 
that is, if strong and homogeneous shadows are required. The 
development, though completed in a few seconds at a high 
temperature, may take half a minute or so at a temperature of 
100 deg. Fahr. This is a very important point, and is not 
sufficiently appreciated, many appearing to suppose that when 
the whole of the detail is visible, no further action can 
follow. 

The so-called " developer " of potassic oxalate is merely the 
vehicle or means of bringing the ferrous oxalate within 
chemical reach of the platinous salt ; this it does by dissolving 
the ferrous oxalate, whereupon the latter immediately reacts 



280 PRACTICE OF PHOTOGRAPHY 

upon the platinous salt, producing metallic platinum and oxi- 
dation of the ferrous oxalate into ferric salts, thus : 

SKsPtCl^+eCgFeO^ 
=Fe3Cl6+2C6Fel20i3+6KCl+3Pt. 

The ferric oxalate formed by ether with that from the 
parts not acted on by light, combines with potassic oxalate, 
forming the apple-green crystals of the donble salt, potassic 
ferric oxalate, which crystallize out from a concentrated cold 
solution of potassic oxalate. 

CeFe30i3+3C2K304=2CeFe'"K30i3. 

The bath of potassic oxalate must never be allowed to be- 
come alkaline from any cause, a possible cause being over- 
heating of the dry oxalate on the sides of the dish, or else- 
where, by which means carbonic oxide is driven off and potassic 
carbonate results. 

An alkaline bath will produce muddy prints ; on the other 
hand, it should be only slightly acid ; in fact, nearly as possi- 
ble, neutral. Still alkalinity is far more to be feared than 
acidity. 

It only remains to be said that the developed prints must be 
washed in two baths of a weak solution of hydrochloric acid to 
clear them. This solution is made by mixing one part of hy- 
drochloric acid with 80 parts of water. 

As soon as the prints have been removed from the develop- 
ing dish, they should be immersed face downward in the first 
bath of this acid, in which they should remain ten minutes, or 
rather less ; they should then be removed to the second bath, 
in which they should remain for a like period. While the 
prints remain in these acid baths they should be moved so that 
the solution has free access to their surfaces. The second 
bath should remain colorless; when it is in the slightest 
degree colored it should be discarded and a fresh one sub- 
stituted . 

On no account should the prints be placed in plain water on 
leaving the developer. 

After the prints have passed through the two changes of 
acid, they should be rapidly rinsed, and then well washed in 
two or three changes of water during about half an hour. They 
are then finished. 

The object of this washing in acid and water is to remove 
the iron salt with which the paper is sensitized. 



CARBON PRINTING. 281 

These prints being on plain paper are better dried across 
glass rods or tubes. If wooden rods or strings are used for the 
purpose they should be kejDt very clean, otherwise stains are 
likely to occur. 

Section IV. 

CARBON, OE FIGMENT PRINTING. 

One of the greatest drawbacks to photography has been the 
great liability to fading of its products. Many and earnest 
have been the efforts to render silver photographs permanent ; 
but all of no avail, for by contact with air they must sooner or 
later fade away. Photographers have for a long time been 
painfully aware of this fact, hence the many attempts to rescue 
the art from the stigma of evanescence. For this purpose car- 
bon or other suitable pigments are now to a considerable ex- 
tent employed in printing. 

It was discovered many years ago by Mr. Mungo Ponton, of 
Edinburgh, that certain organic substances, notably solution of 
gelatine, when treated with a chromic salt, such as bichromate 
of ammonia or potash, spread over paper or other medium, 
dried and exposed to light, are no longer soluble in warm 
water. But to Mr, William Blair, of Perth, is due the dis- 
covery how to produce carbon prints in half tone by develop- 
ment from the back of the sensitive film. Light alone exer- 
cises no decomposing influence on these salts until they are 
brought in contact with the organic body. The chromic acid 
is reduced to a lower oxide of chromium, and the liberated 
oxygen unites with the gelatine which is oxidized into an in- 
soluble resinous substance, or only soluble in proportion to the 
duration and force of the actinic rays. 

Preparation of the Pigmented Paper. — This can be best 
done by those who make a specialty of the manufacture and 
have abundance of space for drying the product uniformly. 
It may be stated generally that the operation is conducted on 
the jack-towel system, that is, a long web of paper is fastened 
round two rollers placed at a distance apart. The lower roller 
is so arranged that the surface of the paper just touches a 
warm solution of pigmented gelatine in a trough. The pig- 
ment may consist of carbon, either alone or with an admixture 
of alizarine to produce the popular photographic color ; but 
pigments of any nature or color may, if inert, be employed. A 
crank on one of the rollers is gently turned until the whole 



282 PEACTICE OF PHOTOGKIpHY. 

sheet has been covered on the outer side. The rollers are then 
raised or the trough lowered, whilst the crank is kept gently 
moving to enable the gelatine to set uniformly. After this the 
web is cut and hung up to dry. 

Sensitizing the Pigmented Paper. — It is of considerable 
importance not to use a strong solution of bichromate for sen- 
sitizing, because when the gelatine is dry, crystals forming on 
the surface are apt to interfere with uniformity of printing, 
and make the tissue so sensitive as to be unmanageable. 

Bichromate of potash, 1 ounce. 

"Water, 1 pint. 

Liquor ammonia, 20 minims. 

During hot weather let the proportion of water be increased. 
A sufficient quantity is made to fill a flat dish to the depth of 
at least half an inch. The paper is cut into sizes rather smaller 
than the dish, which may be of wood, porcelain, or zinc. A 
piece of paper is slipped, pigment side downward, under- 
neath the liquid, and instantly turned over, taking care to 
see that it is covered with the solution, and that there 
are no air-bubbles on the surface. If any appear they must 
instantly be removed, either by a touch of the finger or a brush. 
The paper is almost certain at first to curl up above the solu- 
tion, from unequal expansion ; but this must be prevented by 
keeping it underneath, either by the hands or by means of 
brushes. 

The temperature of the room in which the sensitizing opera- 
tion is performed is of importance ; for gelatine, soluble in 
warm water is more readily so in a heated place. Under or- 
dinary circumstances, in this country, a temperature ranging 
not over 60 or 65 degs. Fahr., can be readily obtained. The 
pigmented paper, as usually prepared, will bear this degree of 
heat without the gelatine slipping off, unless from prolonged 
immersion in the bichromate bath. Should there be a tend- 
ency to solution of the gelatine in hot weather, the plan of 
cooling the bichromate solution with ice and immersing the 
paper for a minimum of time proves an antidote. 

As a rule the paper is sufficiently sensitized when it becomes 
pliable. A longer immersion in the bichromate bath does no 
harm farther than rendering the paper much more sensitive 
and consequently unmanageable. Hemove superfluous moist- 
ure by the squeegee. 

If the sensitized sheets are small, they may be hung up to 
dry in a dark room by an American or glass clip attached to a 



CARBON FEINTING. 283 

string strethed across the room. If tliej are 10x8 inches, or a 
little more, two clips will be necessary, one at each of the up- 
per corners. If the sheets are still larger this is not a safe 
course of procedure, because their very weight would tear 
them away. Under such circumstances the suspension is best 
carried out in the following manner. A number of thin deal 
laths, about three quarters of an inch in width, and sufficiently 
long to stretch more than across the whole breadth of the 
sensitizing dish should be provided. One of these is laid 
across the end of the bath, resting on the sides. As soon as 
the paper is sensitized, the corners nearest tiie lath are seized 
by the forefinger and the thumb of each hand, and the end of 
the sheet placed upon the lath. The other lath is placed over 
this, and both gripped together with a couple of American clips. 
By these handles the sheet is raised gently from the bath, and, 
after allowing the larger portion of the liquid to drain off, is 
drawn over a glass rod. It is then suspended in a cool and dry 
dark room between horizontal bars, on which the ends of the 
laths rest edgeways in grooves. After the paper has again 
drained for a short time, another lath is attached to the bottom 
by clips. The object of this is to prevent the sheet from curl- 
ing up as it dries. Too slow drying conduces to insolubility of 
the gelatine ; too rapid drying to its reticulation. 

The same bichromate bath may be used frequently, and ap- 
parently is not weakened ; but it is gradually changed in some 
way which practically am^rmts to the same thing. And this 
is not to be wondered at wlien we bear in mind the singular 
reactions which take place when chromic acid and some organic 
bodies are brought together. 

When the gelatine tissue has been sensitized, the sooner it 
is dried and used up the better. It should be printed on 
within twenty-four hours if possible ; but if kept thoroughly 
dry, it will remain useful for several days, especially if a weak 
solution of bichromate has been used for sensitizing. 

Printing the Picture. — To those accustomed to silver print 
ing, this will at first appear a difficult operation, but in reality 
it is not so after one has gained a little experience. The rea- 
son why it appears difficult is because no visible impression is 
made on the tissue. Various kinds of actinometers or meas- 
ures of chemical light have been devised, none of which ap- 
pears to the writer to be so satisfactory and simple as a 
piece of silver- chlorized paper sensitized on a standard solu- 
tion. The mode of using this actinometer will be explained as 
we proceed. 



284 PBACTICE OF PHOTOGRAPHY. 

The negatives, after examination by transmitted light, can 
be classified into four different degrees of printing density. 
On a corner of the glass is scratched with a diamond 1, 2, etc., 
as the case might be. The register will serve for future print- 
ings from the same negative, but if a mistake in estimating the 
density, has, after trial, found to be committed, it can of 
course be easily remedied afterwards by altering the figure. 
After a little experience, few mistakes will be committed in 
this respect. 

Before printing on gelatine from negatives a precaution 
should be adopted, which is, however, not always necessary^ 
A narrow strip of blackened paper is gummed all round the 
edges of the negative ; because it has been found that when 
liglit has acted strongly, the changed gelatine will not adhere 
to the transfer paper with sufficient tenacity to resist the 
action of the warm water, which insinuates itself underneath,, 
and may destroy the picture. This bordering has been hap- 
pily called the safe edge. An edging of black varnish answers 
equally well. 

The plan which the writer adopts for the exposure of the 
sensitive tissue is this : A dozen, say, ordinary printing 
frames are filled in the dark room. On each frame is a chalk 
mark representing the density of the negatives contained 
therein. All the frames are exposed to light, as near as may 
be simultaneously, and at the same time a bit of the silver- 
chlorized paper is subjected to the same actinic influence. 
When this paper registers, to the eye, a certain amount of 
darkening, all the frames marked 'No. 1 are turned over ; when 
it registers a deeper color, the JSTo. 2 set are similarly treated ;. 
and so on, until the whole are supposed to be fully exposed.. 
It may be stated, generally, that the time of exposure, with 
paper prepared as above directed, is about one-third of that, 
required for printing from the same density of negative on 
the most sensitive silver paper. Thus, an operator who has 
been accustomed to the latter mode of procedure, may make 
an excellent actinometer for his own guidance, by simply 
exposing a piece of standard silvered paper under a negative 
in the printing frame, and judging, from an occasional exam- 
ination of that, as to the time in which the gelatine will be 
properly impressed. 

Development of the Image. — In order to obtain a non- 
reversed print from an ordinary negative, the exposed tissue 
must be attached to a temporary support during development. 
For this purpose, a zinc, glass, or other plate slightly rough- 



CARBON PRINTING. 285 

ened may be employed, and, to prevent the image from 
adhering too closely, so that it cannot afterwards be removed, 
the surface of the plate should be smeared with a little wax 
and resin dissolved in turpentine. The excess of this is after- 
wards rubbed off with a clean rag. A quantity of these plates 
are prepared corresponding to the number of the prints to be 
■operated on. 

The next operation is to attach the exposed gelatine to this 
support. Let us follow the progress of one example, which 
will explain all. The print is immersed in a tray containing 
cold and clean water. In a short time, it has absorbed suffi- 
cient moisture to make it adhere firmly. When first im- 
mersed, its tendency is to curl inwards. After a time, it will 
become flat and then curl outwards. The time for removal is 
just before it becomes flat. The plate is now slipped under 
the water, the gelatine side of the tissue pressed closely against 
it, and the whole drawn out in such a way as to avoid any 
possibility of intervening air bubbles. Perfect adhesion is 
effected by expelling the superfluous moisture with a long slip 
of india-rubber inserted between two boards. This instru- 
ment is generally called a "squeegee," and is scraped with 
some pressure over the back of the attached print so as to 
expel as much water as thus can be driven out. After a few 
minutes, or longer, provided the tissue is not allowed to 
become dry, the development proper is commenced. 

The plate, with the paper attached, is placed in a dish of 
warm water registered from 90 deg. to 100 deg. Fahr., and 
remains there for a minute or two until the colored coating 
begins to exude from the edges. Raise up gently one of the 
corners of the paper, and it will leave the plate easily. Should 
it not do so, allow longer soaking; for, if violence be used, in 
all probability the print will be destroj'ed. The paper when 
removed is worthless, and is thrown away ; the greater part 
of the gelatine has left it, and the picture, hidden amidst 
pigment, will be found attached to the zinc or other support- 
ing medium. The plate is at once replaced in the warm 
water, when, with gentle agitation, gradually the soluble gela- 
tine with its contained pigment is washed away and the 
picture appears. Now is the time to judge whether the 
exposure has been properly timed. If the image appears 
faint, the exposure has been too short. In this case, there is 
no remedy. On the other hand, if the details are too dark, 
hotter water will often clear them up. There is great room 
for judicious management in this respect. A tinal rinse in 



286 PKACTICE OF PHOTOGRAPHY. 

cold water removes any loose particles of pigment that may 
be lying on the surface, and sets the gelatine. 

The next step is to transfer tlie developed print from its 
temporary support on to the final one. For this purpose 
"transfer paper " is required. Ordinary paper is too porous 
to admit of being used, but it is fitted for the purpose by 
being coated with a thin layer of gelatine in which a little 
alum has been dissolved. The object of adding the alum is 
to convert the gelatine when dry into a substance analogous 
to leather, and hence to render it insoluble in even very hot 
water. A piece of this paper, rather larger than the devel- 
oped print, is immersed in warm water for a minute or two 
until it feels slimy, and is then at once laid on the glass or 
other plate on which the image was developed, avoiding air 
bubbles. The "squeegee" is again applied to expel super- 
fluous moisture and ensure close contact. When the paper is 
dry, but not till then, it is stripped from the plate, when it 
will be found that the picture has left its first support and is 
imbedded as it were in the paper. Occasionally it may hap- 
pen that the print even when dry exhibits a reluctance to 
leave the supporting plate. This arises from its having been 
imperfectly waxed. A gentle heat may be applied to the 
back of the plate. 

Should any wax or resin be visible on the surface of the 
finished print, it may be rubbed off with a tuft of cotton wool 
impregnated with benzole. 

The above instructions refer to what is called the " double 
transfer" process, whereby the pictures are seen in their natural 
position, that is, non-reversed. But the impressed gelatine 
tissue can at once be attached to its permanent support and 
developed thereon. This is called the "single transfer" meth- 
od, which is much simpler in details, and would be univer- 
sally adopted were it not that in printing from a negative on 
glass the transferred image is reversed, that is, the right-hand 
side of the picture is placed to the left, and vice versa, just as 
in a daguerreotype or a collodion positive on glass. To obvi- 
ate this palpable drawback to an exceedingly simple mode of 
printing, the collodion film may be removed from the glass 
and the negative preserved in a pellicular form, by which plan 
it can be printed from either side without any sensible loss of 
definition. One side would be placed in contact with silver- 
chlorized paper, and the other with sensitized gelatine as the 
case may be. To remove the film from the glass, first dis- 
solve off" the varnish with alcohol, then apply gum water, fol- 



CAEBON PRINTING. 287 

lowed wlien dry by an application of thick collodion contain- 
ing castor oil. When dry the whole may be detached as 
a pellicle. Immersion in water is sometimes necessary to 
aid in detaching it. 

Photographing on Wood Blocks. — Carbon pictures may be 
made upon wood blocks for the use of the engraver. The 
block is smoothed in the usual way. A weak solution of 
gelatine in warm water containing a little alum is made. The 
surface of the block, also warmed, is rubbed over with a little 
of this, and all excess wiped away with a clean cloth. The 
exposed pigmented tissue, after lying for about a minute in 
cold water, is laid on the block, which is instantly pressed 
down on a smooth flat surface, such as glass, to expel air bub- 
bles and secure perfect contact. The block is then set up 
edgeways for a short time, till the tissue is ready for develop- 
ment, which is effected in a manner not materially different 
from that already described. The block is held in the hand 
so that the surface and no more touches the warm water. 
After a little time the paper tissue is removed and the image 
developed by holding the block in the same way. 

A few precautions are necessary. First, the pigmented tis- 
sue should be of that kind which has a minimum of gelatine 
and a maximum of coloring matter. The object of this is to 
prevent the graving tool from slipping while drawing the fine 
lines. Second, immediately before developing, the back of 
the block should be moistened to prevent unequal expansion 
and probable cracking of the wood. 

Photographs laid down in this way, so far as position is 
concerned, are exactly what the engraver requires, viz., a re- 
versed image. But, on the other hand, he has no lines to 
guide him. 

Carhon Transparencies. — Transparencies on glass, for use 
in the stereoscope, the magic lantern, or for window decora- 
tion, may very easily be made by the carbon process. 

The printing must be carried a little deeper than is the case 
when paper is to be the final support ; and such preparations 
as are made to facilitate the image being transferred from the 
glass to paper are here omitted, the most perfect adhesion be- 
ing required. 

The carbon tissue, previously cut to the dimensions of the 
glass, is printed as previously directed, with a very narrow 
safe-edge ; it is then " squeegeed " down on the glass and de- 
veloped. The picture is then immersed for a few seconds in 
a saturated solution of alum, washed, and allowed to dry. 



288 PRACTICE OF PHOTOGRAPHY. 

Solar Enlargements. — The carbon process as applied to the 
production of enlargements of every kind is one of exceed- 
ingly great value. Its use with the solar camera has been 
much facilitated by the discovery that the insolubility started 
by the action of light may be continued in total darkness in 
presence of a slight degree of moisture in the atmosphere. 

This discovery permits of a comparatively brief exposure 
being given in the solar camera, followed by the further de- 
velopment that goes on when the partially impressed tissue is 
suspended for a period of a few hours, or even days, in a dark 
cupboard. It is important to notice that this continuating ac- 
tion which goes on in the dark depends upon moisture, for in 
a dry atmosphere less progress will have been made after sev- 
eral weeks than will, when the atmosphere is moist, be made 
in a few hours. Those who desire to avail themselves of this 
property should have the moisture in their dark cupboards 
regulated by means of a hygrometer or a dry and wet bulb 
thermometer. 

Further Applications. — Premising that the carbon process 
is applied to opal glass, canvas, ivory, and, indeed, to every 
kind of surface, we must refer our readers for further details 
to the " Autotype Manual," which is devoted to this subject. 

Section Y. 
photo-lithography. 

Many attempts have been made to render this process capa- 
ble of reproducing photographic half-tone, but all have been 
of no avail, or at the best have succeeded only very partially. 
The cause of failure lies in the fact that only decided lines 
will allow the greasy ink to bite or take hold of the stcnie. 

Supposing that a map or any other outline, no matter how 
complex, has been drawn, and that it is deemed desirable to 
reproduce faG-similes or reduced copies by means of the litho- 
graphic press, the picture is fastened up flat and a negative 
taken in the camera by a non-distorting lens. The negative 
must be clear and bright in the outline of shadow, and quite 
opaque in the parts represented by white in the original. In- 
tensifying agents in the development are used for this latter 
purpose. 

The following is Mr. ()sboi;ne's mode of ])rocedui'e : 

A sheet of plain positive photographic paper is now coated 
on one side with a mixture consisting of gelatine softened and 



PHOTO-LITHOGEAPHY. 289 

dissolved in water, to which a quantity of bichromate of 
potash and albumen lias been added. The paper,, evenly cov- 
ered with this fluid, is dried in the dark, when it will be found 
possessed of a smooth, glassy surface, and a brilhant yellow 
color. This surface is still further improved by passing it 
through the press in contact with a polished plate. 

A suitable piece of positive photo-lithographic paper thus 
manufactured is now to be exposed to the action of light 
under the negative of the map already described. This is 
accomplished in an ordinary pressure-frame, the time required 
varying from 10 to 15 seconds to several minutes, accord- 
ing to the brightness of the weather ; but it is always short 
compared with that necessary for the production of a picture 
on paper prepared with chloride of silver. The positive thus 
obtained presents itself to the eye as a brown drawing upon 
the clear yellow of the sheet. If the prepared surface of the 
paper were now moistened with water, and the attempt made 
to apply printing ink to it, we would find a strong tendency 
in the albumino-gelatinous surface to behave towards greasy 
and watery substances in a manner quite analagous to that 
peculiar to a lithographic stone while printing. "We would 
also find that the solvent action of water at any temperature 
is quite incapable of removing the picture which the sun has 
imprinted upon it. The light, in fact, has so acted upon the 
chemical substances brought together upon the surface of the 
paper that the organic matter is no longer soluble. These are 
characteristics of the change due to exposure which we have 
to remember. 

But the exposed photographic copy of the original is not 
moistened or subjected to any solvent action at this stage of 
the proceedings ; it is, on the contrary, covered all over while 
dry with lithographic transfer ink, which is accomplished by 
running it through the press with its face in contact Avith a 
stone which has already received a coating of such ink. After 
it is separated from the blackened stone, it will be found to 
have brought away with it an evenly distributed film of inky 
matter, forced by the pressure into intimate contact with the 
unexposed as well as the exposed portions of the surface. 
This operation is known as " blacking " the positive print. 
That now to be described is called " coagulation," its object 
being to effect a change of that nature upon the albumen con- 
tained in the coating of organic matter. For this purpose, 
moisture and heat are necessary, and both are applied very 
simply, by letting the blackened photographic copy swim 



290 PEACTICE OF PHOTOGRAPHY. 

npon the surface of boiling" water witli its inky side upwards, 
for it is important not to wet that with hot water. After the 
lapse of a certain period, determined by the experience of the 
operator, lie proceeds to the next step in the process, that of 
"washing off." For this purpose, the print is laid upon a 
smooth surface, such as a plate of glass or porcelain, and 
friction with a wet sponge or other suitable material is 
aj)plied to the black, inky coating under which the photo- 
graphic image still exists, and to develop which is now the 
object in view. The operator soon becomes aware that the 
moisture which percolated through the paper from the back 
has exerted a softening influence upon the gelatine in the sen- 
sitive coating ; it has caused it to swell and let go its hold 
upon the ink. But this change does not extend to those parts 
of the coating which were acted on by light ; in other words, 
to those places which were unprotected by the opacity of the 
negative ; they remain intact, uninfluenced by the solvent or 
moistening effect of the water. Accordingly, the operator 
finds a fac-simile of the original map gradually develop under 
his hand as he continues the friction. This process is pro- 
ceeded with till all traces of ink ai-e removed, save those re- 
quired to form the picture, which must be clear and distinct in 
all its details. Abundance of hot water is then poured over it, 
so as to remove every particle of soluble matter, and then it is 
finally dried, which completes its preparation. We are now 
possessed of a photograph in lithographic ink, identical in 
every respect with the original, not simply upon paper, but 
upon albumenized paper — a matter of much importance as will 
presently be explained. The presence of the albumenized 
layer under the ]3icture is the result of the coagulation which 
took place while the print was swimming on the hot water ; 
after that change no amount of washing could remove it, al- 
though the gelatine was not proof against such treatment. 

A stone to which a fine smooth surface has been imparted is 
now slightly warmed and put in the lithographic press. Upon 
this is placed inverted the positive print, after it has been 
damped by lying between moist paper, and the whole is then 
passed repeatedly through the press. On examination the 
paper will now be found to have attached itself firmly to the 
stone, so that some force ' is required to separate the two. 
When the former is removed it brings with it the albuminous 
coating, which gives to it while damp a parchment-like ap- 
pearance. But the ink is gone ; it has left the paper for the 
stone, and on the latter we find a reversed drawing of the map, 



PHOTO-ZINCOGRAPHY. 291 

one wliich, after it has been properly "prepared," will print as 
well as if it had been drawn by hand. The rationale of this 
method of transfer is easily understood ; a greasy ink having a 
great affinity for the substance of the stone, combines with it 
to form a lithographic drawing in the strictest sense of the 
word, and while this is taking place the damp albumen upon 
the paper holds the sheet in its proper place, so as to prevent a 
shift of any kind, and enables the pressure to be applied as 
often as the operator wishes. 

The stone is printed from in the usual lithographic way. 

Section YI. 
photo-zincogeaphy. 

This process in principle is the same as photo-lithography, 
but some of the details are different. It was invented by 
Colonel Sir Henry James, and is now extensively employed in 
the Ordnance Survey Department at Southampton for copying 
maps, etc. 

A suitable paper is floated for two or three minutes on a 
warm solution (about 100 deg. Fahr.), of the following sub- 
stances : — Bichromate of potash, 2^ ounces, dissolved in 10 
ounces of hot water, to which are added 3 ounces of the 
purest gelatine previously dissolved in 40 ounces of hot water. 
The paper, after becoming dry, should be again floated on the 
same solution, and hung up to dry at tlie opposite corner to 
that by which it was first suspended, in order to distribute the 
sensitizing solution uniformly. This must be done in the dark 
room. This paper will not keep long in a serviceable state 
even in the dark room, because the bichromate gradually 
oxidizes gelatine without the action of light. Two days are 
about the limits of its keeping qualities. 

The sensitive paper is exposed to the solar rays under a 
negative in the pressure-frame as usual. One minute in bright 
sunlight is often sufficient. The general indications to jndge 
of sufficient exposure, are the appearance of the parts where 
the light has acted most strongly. They should be of a deep 
tawny color tinged with green, and the shadows yellow. 

Now comes that part of the process where the lithographer 
steps in to complete the work of the photographer. The 
print is removed from the pressure-frame and inked l)y the 
following method : 



292 PRACTICE OF PHOTOGRAPHY. 

In an iron pot put 2 ounces of Burgundy pitcli, 1 ounce of 
palm oil, and 1 ounce of bleached beeswax ; place the pot 
over a fire, and as soon as they begin to melt, keep stirring the 
mass till they are thoroughly incorporated, which will not take 
place till the ingredients have neai-ly reached the point of 
ignition. Then remove the pot from the fire, and intimately 
mix with the contents 1 pound of chalk, lithographic ink, and 
half a pint of what is called in the trade middle linseed oil 
varnish, both of which must have been previously thoroughly 
incorporated by pounding in a mortar. 

When required for use, a portion of the ink is melted with 
suflScient turpentine to make it of the consistence of honey. 
A little is then placed on a printing roller, and a flat zinc plate 
inked with it in the usual manner. The print is then laid 
face downward on the zinc, and the whole passed through a 
press, by which means it receives an even coating. 

The print is then removed from the zinc plate, and laid 
back downward on water at the temperature of about 100 
deg. Fahr. for a few minutes. It is next placed on a level 
slab, and all the superfluous ink removed with a soft sponge 
dipped in gum water. It is afterward treated with repeated 
baths of warm water till the ground is quite clear. When 
dry, it is ready for transferring to zinc or stone. 

Mode of Transfer^ c&g. — Colonel Sir Henry James' instruc- 
tions on this part of the process are so very lucid and precise 
that we cannot do better than quote them. 

The Transference of the Print to Zinc, and Preparation for 

Printing. 

When the zinc plates are received from the manufacturer, 
tlie surface has to be prepared to receive transfers. They are 
first planed with a razor blade, the back of which is set in a 
wooden handle, the ordinary edge is ground down fiat so that 
there are two edges to scrape with in turn, like the edges of a 
skate. The plate is thus cut down till all surface scratches, 
blisters and other defects are obliterated. It is then ground 
down to a fiat surface with pumice stone, and smoothed with 
snake stone, to take out any scratches made by the pumice 
stone, Finally, a grained structure is given to it by rubbing 
with fine sand and water and a zinc muller. The muUer is 
simply a disk of zinc, about half an inch thick and four inches 
in diameter, fixed to a wooden handle. It is grasped by the 



PHOTO-ZINC^GRAPHT. 293 

handle with the thumb uppermost, and rubbed over the sur- 
face of the plate with a circular movement. 

The sand is brought to the requisite degree of fineness by 
sifting it through a wire sieve of from 80 to 100 holes to the 
square inch, according to the kind of grain required for the 
plate. The time required for two men to grain a zinc plate three 
feet long by two broad has been found to be about an hour. As 
soon as this process is completed, the plate is thoroughly 
washed with water and well dried. It should be kept from 
contact with any substance likely to communicate greasiness to 
it ; and the sooner it is used for transferring the better, as the 
action of the atmosphere will tend to diminish the afiinity of 
the surface for the greasy ink. 

When it is desired to clean and prepare for receiving trans- 
fers a plate which has been used, the ink of the old transfer is 
cleared off with turpentine, the plate is then washed with 
strong alkali and cleaned with water ; an acid is then poured 
over it. This is prepared by taking equal parts of sulphuric and 
hydrochloric acids, and to 1 part of the mixture adding 12 
parts of water, and the plate is regrained in the manner al- 
ready described. 

The photographic print is laid between sheets of damp pa- 
per for a few minutes, placed face downward on the zinc 
plate, witli two or three sheets of paper over it, and passed 
through the press. 

If the transfer print is not more than three or four days old, 
it will be sufficient to pass it through once, but an old print on 
wh ch the ink has had time to harden will require to pass 
through the press two or three times. 

Tlie sheets of paper covering the transfer are then removed, 
and it is damped with a wet sponge for 2 or 3 minutes ; 
this causes the gelatine in the lines to swell, and makes the 
ink leave them more readily. 

The print is then pulled carefully off the plate, and nearly 
the whole of the ink should remain on the zinc. 

The transfer is now etched ; the etching liquid consists of a 
decoction of galls and a little phosphoric acid, mixed with a 
thick solution of gum and water. 

It is prepared as follows : 

Four ounces of aleppo galls are bruised and steeped in 
3 quarts of cold water for 24 hours ; the water and galls are 
then placed in a vessel over the fire and allowed to boil up. 
This decoction is then strained. The gum water should be 
about the consistence of cream. 



294 PRACTICE OF PHOTOGRAPHY. 

One quart of the decoction of galls is added to 3 quarts of 
gum M^ater, and to the mixture is added about 3 ounces of the 
solution of phosphoric acid, which is prepared by placing 
sticks of phosphorous in a pint bottle of water. This is stop- 
ped with a cork, in which is cut a small hole ; the bottle is 
three-quarters filled with water, and the ends of the sticks of 
phosphorous rise above the surface and become oxidized by 
the air admitted into the bottle. 

The phosphoric acid, as fast as it is formed, is dissolved by 
water. In a few days, the solution is strong enough for use. 

The etching liquid is poured on the plate, and wiped over 
the surface with a sponge or camel's-hair brush. It is allowed 
to remain on for a short time, varying with the strength of the 
design. With fine work, 20 seconds would be sufficient. 
Strong lines will bear the action a minute without injury. 
As soon as the solution has acted sufiiciently, it is wiped with 
a soft cloth dipped in water, care being taken to remove all 
trace of it if there are fine lines. 

The transfer ink is next cleared from the zinc plate with 
turpentine ; or, if the design is weak, with turpentine mixed 
with olive oil and gum water. It is then rolled up with print- 
ing ink, the roller being very thinly and evenly coated. 
Impressions can then be printed in the usual manner ; 1,500 
is not an unusual number for the plate to stand without sen- 
sible deterioration. 

The photographic print can be transferred to a lithographic 
stone in a similar manner. 

When the subject admits of it, paper enameled with zinc 
white should be used, as the impressions produced are most 
perfect. 

It is prepared in the following manner : 

Four ounces of Russian glue are soaked in 3 quarts of water 
for some hours, and then heated till dissolved ; a pound and a 
half of zinc white is ground with water on a slab, and then 
mixed gradually with the solution of glue and passed through 
a hair sieve. 

A coating is brushed on the paper with a pound brush, and 
the streaks are obliterated by going lightly over the surface 
with a flat camel's-hair brush. A second coating is applied in 
a similar manner, and hung up to dry. When dry, it is ready 
for use. 



PHOTO-EELIEF' PRINTING. 295 

Section VII. 

woodbuey's process ; oe, photo-eelief feinting. 

Any good negative suits for this process, but the details of 
manipulation are not very easy. 

First Stage : Preparing the Tissue. — A clean glass plate is 
smeared over with beeswax, which is then rubbed with a clean 
cloth till the coating is infinitesimally thin, A strong-bodied 
and tough collodion is poured over the waxed side of the 
glass in the usual way, and allowed to dry. The plate is 
coated on the collodion side with a thick and warm aqueous 
solution of gelatine, containing from 15 to 30 per cent, of 
bichromate of ammonia or potash, and is then laid on a level- 
ing stand, in the dark room, to set in a uniform sheet. 
When the film has set, the plate is put into a closed drying 
box containing fused chloride of calcium, which soon desic- 
cates the gelatine by absorbing the moisture. The compound 
film of collodion and gelatine can now be raised from the 
glass in one unbroken sheet by cutting round its edges with a 
sharp knife and gently raising it up. With as little delay as 
possible the tissue is subjected to the next operation. 

Second Stage : Impressing the Tissue. — The collodion side 
of the tissue is placed in contact with a negative in a common 
printing frame, and subjected to the direct solar rays, which 
should fall perpendicular to the surface of the glass, otherwise 
the image will lose much of its sharpness, for the following 
reason : As the actinic impression has to be made through an 
appreciable thickness of insensitive collodion intervening be- 
tween the surface of the negative and the sensitive gelatine, 
and has also to penetrate through the latter to some consider- 
able depth, any parallax or change of relative positions of the 
different parts of the negative, arising from various obliqui- 
ties of the light, would seriously affect sharpness of definition, 
and, to some extent, truthfulness of delineation. The above 
remarks will be appreciated by those who, by inadvertence, 
have made a print from the wrong side of a negative on glass. 
The image is all confused. But if any one tries the experi- 
ment of printing from the wrong side of a negative on very 
thin glass, by placing and keeping it quite perpendicular to 
the sun's rays, at the same time excluding, as far as possible, 
all diffused light, he will find his princ nearly, if not quite, as 



296 PRACTICE OF PHOTOGRAPHY, 

fine as if the sensitive paper had been in immediate contact 
with the negative image itself. 

The above are the principles which must be attended to in 
impressing the sensitive gelatine film. Artificial light from 
the charcoal points of a powerful magnetic battery may be 
used ; but in this case, as the printing frame must be placed 
near the source of light, there will be considerable parallax. 

Third Stage : Making the Mould. — When the exposure is 
completed, the unimpressed gelatine is washed away. To do 
this, the collodion side of the tissue is pasted down flatly with 
a benzolic solution of india-rubber on a piece of clean glass, 
and the edges varnished. The whole is placed in a dish of 
warm water, which dissolves oft' all the gelatine where light 
has not acted, and the rest exactly in proportion to the actinic 
impression, leaving a depressed surface in the high lights cor- 
responding to the densest portions of the negative, and a 
gradually increasing relief through all intermediate tones up 
to the deepest shadows. The film still attached to the glass is 
dried, and can then be easily detached in one unbroken sheet 
by cutting round the edges and gently raising it. 

At this stage the value of a little Prussian blue, mixed with 
the bichromated gelatine, is obvious. Pure gelatine, when in 
thin stratum, is transparent, and the inequalities on the raised 
surface corresponding to the lights and shades of the negative 
could hardly be appreciated without some pigment being 
mixed with the gelatine. Prussian blue is the best to use, be- 
cause it offers little obstruction to the chemical rays during 
exposure. 

The mould is now complete. 

Fourth Stage : Making the Printing Die. — For this pur- 
pose a composition of lead and type-metal is used. On a block 
of this composite metal, about a quarter of an inch thick, the 
collodio-gelatine mould is laid, gelatine side downward, 
placed in a hydraulic press, and subjected for a few minutes 
to a pressure of over a hundred tons. The result is a sharply 
defined and re versed-relievo image on the metallic surface. 
The high lights are, of course, now raised and the shadows 
depressed. Strange as it may appear, the gelatine mould, al- 
though thus squeezed witli such enormous pressure against a 
metallic body apparently harder than itself, has not suffered 
in the least, and may be used for taking many similar impres- 
sions, all as sharp as the first one. 

The metallic die is now trimmed to the size of picture re- 



PHOTO-EELIEF' PRINTING. 297 

quired and beveled on the upper edges, when it is ready for 
casting out the bas-relievo proofs on glass, paper, or other 
suitable medium. 

Fifth Stage : Casting or Printing from the Die. — To get 
proofs on glass and other rigid media, the die is placed on a 
level slab and rubbed over with a little oil. A small pool of 
aqueous solution of gelatine, impregnated with carbon or any 
desired pigment, kept ready at hand in a warming apparatus, 
is poured on the middle of the die. Immediately the glass or 
other plate, also warm, is pressed thereon with the hand for 
an instant, and left in position till the gelatine sets, which 
might be in about two or three minutes. After that, the 
glass or other medium, with the picture firmly attached to it, 
may be removed, and the superfluous pigment round the 
edges scraped off. The whole is then varnished. As one 
man can work several dies, and as the die requires oiling only 
once for five or six impressions, the prints can be turned out 
at the rate of about one per minute. 

When paper or other flexible porous medium is used to re- 
ceive the picture, the process of printing is slightly different. 
The paper must first be impregnated with some substance which 
renders it non-absorbent, else some of the pigment would be 
squeezed into the pores of the paper instead of being pressed 
away from the parts of the die in highest relief. A pressure 
frame is also required to press the pliant medium in close con- 
tact with the pigmented die. 

The rationale of the final casting on glass, paper, etc., of 
this ingenious process, is very easily understood. The die, or 
metallic impression from the gelatine mould, is something 
analogous to a queen's head on a coin, but not in such high 
relief. The most elevated portions of the die corresponding 
to the high lights of the picture squeeze away the whole of 
the pigment, and those less elevated press it out in proportion 
to their greater or less elevation. Thus perfect gradation of 
tone is secured. 

Two or three drawbacks to the extensive use of this process 
exist. First, the die cannot be printed from along with letter- 
press. Second, no means have been devised whereby the 
crushed out pigment should not smear the borders of the 
paper, etc., on which the picture is impressed. And third, 
the production of large images seems to be an impracticable 
operation. 



298 pea.ctice of photography. 

Section YIII. 
the stannotype. 

The stannotype is closely related to the former process, and, 
like it, is tlie invention of Mr. Woodbury. Its distinguish- 
ing feature consists in the method by which the printing sur- 
face is made. 

A transparency obtained by any suitable process is used as 
a cliche to impress an image on a sheet of carbon tissue, which 
contains a large pro]3ortion of gelatine and but little coloring 
matter. This is sensitized by immersion for fire minutes in a 
six per cent, solution of bichromate of potash. It is then 
dried quickly over chloride of calcium or lime, and then ex- 
posed to light in the printing frame under the transparency, 
a " safe edge," as described in the chapter on pigment print- 
ing, being employed. The image is developed upon patent 
plate-glass, half an inch larger each way than the opening in 
the mask, and the temperature of the water employed in de- 
veloping should not exceed 110 deg. Fahr. 

After becoming dry, which is hastened by first placing the 
plate in alcohol for some time, it will be seen that there is a 
great degree of relief in the picture, which is now intaglio. 
It is then covered with a sheet of tinfoil, either plain or 
coated with a delicate steel layer, and to ensure adhesion it is 
previously coated with a thin solution of india-rubber in ben- 
zole. By passing the glass plate thus prepared between a 
pair of india-rubber rollers, with proper precautions not to 
fracture the glass, and yet to have sufficient pressure applied 
to bring the tinfoil into contact with every part — even the 
deepest indentations of the intaglio — a surface is obtained 
from which impressions may be obtained in the manner de- 
scribed in the previous chapter. 

Section IX. 

PHOTO-BLOCK PRINTING. 

Among various ways of producing surface-printing blocks 
by means of photography, there are two which are much 
employed. 

{a) Gelatine Beliefs. — A plate of glass is coated with bi- 
chromatized gelatine and dried. It is then exposed under a 



PHOTO-BLOCK PRINTING. 299 

negative of any line subject, such as printed matter or engrav- 
ings, and transferred to a dish of cold water. The parts 
unacted upon by light immediately swell and stand in relief, 
leaving the p*i'inted matter flat as before. 

From this relief a cast is obtained by an electro-metallurgic 
process, by plaster of paris, or by one or another of the various 
methods so well known in connection with stereotyping pro- 
cesses, 

{h) Line-Etched Reliefs. — Paper having been sized with 
albumen and bichromate of potash, and carefully dried in the 
dark, is exposed in a printing-frame under the negative. It is 
then blackened all over with lithographic transfer ink, and 
laid face upward in a flat dish of cold water. By applying 
gentle friction to the surface, the whole of the ink is removed, 
witli the exception of that which adheres to the ]3rinted 
matter. 

The transfer is next laid face down upon a smooth and 
clean zinc plate, and passed through a press, by which the ink 
is set off on the plate. The image is then rolled up and 
strengthened in the manner practiced by lithographers, an 
application of powdered resin and bitumen made to strengthen 
its acid-resisting properties, and after a. gentle heat to fuse the 
resin, the plate is placed in a vessel containing greatly diluted 
sulphuric oxhydrochloric acid, by which the unprotected parts 
of the plate, representing the whites, are dissolved away, leav- 
ing the other parts standing in relief. Undercutting of the 
lines by means of the acid is provided against by alternating 
the etching process with occasional applications of the print- 
ing roller, followed by the dusting on of resin and fusing. 
The fine powder applied diagonally by means of a flat camel's- 
hair brush, followed by fusion, answers a similar purpose, viz., 
the protection of the sides of the raised lines from the under- 
mining action of the acid. 

There are other photo-etching processes, some of them based 
upon the sensitiveness to light of. bitumen, and its becoming 
insoluble by exposure, and consequently forming an etching 
ground ; others by a similar property residing in bichromatized 
gelatine, the non-exposed parts of which are permeable by 
solutions of perchloride of iron, or chloride of platinum, while 
the exposed portions are acid-resisting, but the processes we 
have described are those most approved in practice. 



300 PKACnCE OF PHOTOGRAPHY. 

Section X. 

COLLOTYPE PRINTING. 

The method of printing from a film of gelatine by means 
of printers' ink has received various trade designations, such 
as lichtdruch, heliotype process, etc. ^^^^J^^^'l 

The principle on which the process is based is analagous to 
that of photo-lithography, with this important difference, viz., 
that in collotype printing the sensitized gelatine is so changed 
by the action of light that it takes the printers' ink exactly in 
proportion to the actinic impression made. 

Mr. Sawyer, who has been eminentl}' successful in this mode 
of printing, furnishes the following details of manipulation : i.^ 

A mixture is made of gelatine, albumen, and bichromate of 
potash, the ingredients being well beaten together ; when the 
froth has settled down the mixture is filtered. The plate,, 
having been previously leveled in a drying-box, and warmed 
up to a temperature of 100 deg. Fahr., is coated with a por- 
tion of the preparation, which is made to flow all over the 
plate, and a portion over each edge ; the plate is restored to its 
drying-box, and in about two hours or so the first coating is 
dry. The second preparation is composed of gelatine, one to 
six, and a small portion of albumen, and sensitized with a 
bichromate. When these are thoroughly dissolved, they are 
whisked well together, and whilst being agitated a small quan- 
tity of an alcoholic solution of resinous gum is added ; an 
emulsion is instantly formed, the particles of the gum being 
entangled with the gelatine. There is added a trace of nitrate 
of silver, with a small quantity of a solution containing an 
alkaline iodide. The whole, after being well stirred together, 
is filtered ; the plate is again leveled in its drying-box. When 
all is ready the plate is placed in a porcelain dish of warm 
water, the excess of bichromate of the first coating is washed 
away, the heat of the water and the residue of the chemicals 
remaining coagulate the albumen, and produce a very delicate, 
slightly-porous surface admirably fitted for the reception of 
the second preparation, which is poured on the plate whilst 
still moist. Care is taken to let it drive off the superfiuous 
water, and a portion of the preparation itself is allowed to 
escape over each edge, which has the effect of binding down 
the film firmly to the glass. The plate is restored to its drying- 
box, and at a temperature of 90 or 100 deg. Fahr., becomes- 



COLLOTYPE FEINTING. 301 

dry and ready for use in two or three hours. When dry and 
cool a negative is laid down upon the plate-glass of the pres- 
sure-frame, and the plate-glass bearing the sensitive surface 
laid upon it. The progress of the printing can be easily ascer- 
tained by looking through the plate from the back. When 
the picture appears well and all the detail visible, it is done 
enough. After exposure the plates are washed with cold 
water, rinsed thoroughly, and allowed to dry spontaneously ; 
they are then ready for the press. Plates prepared by this 
method have exceedingly thin films, not exceeding the thick- 
ness of writing paper, and to this fact perhaps is due the ex- 
quisite reproduction of line and delicate detail. In thick films 
there is an appreciable and, very frequently, a strong relief, 
requiring enormous pressure to force the paper into it ; 
in these thin films the process is exactly analogous to that 
of lithography, and the pressure required is comparatively 
trifling. 

The plate, when dry, is leveled on the bed of an ordinary 
lithographic press. A little plaster of Paris is run on a litho- 
graphic stone, and the glass plate is laid upon it whilst it is still 
fluid. No plate laid down in this way will ever break, what- 
ever pressure may be applied to it. This plan, however, being 
rather troublesome, the following is usually adopted : — Venice 
turpentine, thickened with a little wax, is smeared on a slab or 
lithographic stone, and the plate is worked down upon it for a 
few seconds ; it soon becomes set, and may be printed from with- 
out the slightest risk — only beware of any grit between the glass 
and the stone. Having in one or other of these ways, got the 
plate upon tiie bed of the press, it is carefully leveled up, and 
well sponged with cold water, dried off with blotting-paper, 
then wiped with a piece of fine soft muslin and ink rolled into 
it with a lithographer's leather roller. If the plate is over- 
exposed, or under-exposed, a judicious use of thicker or thinner 
ink will still produce good results. Sometimes two or more 
inks of the same or varying degrees of stiffness are used in 
color-printing, frequently only one. The operation of print- 
ing is one requiring delicacy, taste, and skill in the rolling ; 
and the reason that the productions of Berlin and Munich are 
so exquisite is that they are able to command the very finest 
trained skill in lithographic printing. 



CHAPTEK XVIII. 

THE GELATINE EMULSION PEOCESS. 

In 1871 was initiated what has proved to be a revolution aiy 
power in the production of dry plates possessing a degree of 
sensitiveness never previously obtained by any process, wet or 
dry. In that year Dr. R. L. Maddox submitted to the editor 
of one of the London weekly photographic journals several 
negatives he had taken by an emulsion of bromide of silver 
with gelatine, instead of collodion, which had hitherto been 
employed. Having to retire for a time from the experimental 
practice of photography, he published the details of his method 
of proceeding in order that others might continue the investi- 
gation. This has been done, with effects of a scientific, ar- 
tistic, and commercial nature which are truly astonishing. The 
wet collodion process has now been banished from a large 
number of operating rooms, for among other properties inher- 
ent in dry gelatine plates is the desirable one of a degree of 
sensitiveness far exceeding that of collodion. This, in addi- 
tion to the convenience arising from having in readiness for 
use at all times a supply of dry plates, which, being supplied 
through commercial sources in every size, of extreme sensi- 
tiveness and that excellence arising from competitive specialite 
manufacture, now enable the photographer to economize his 
labor, dismiss his sitters quickly, and, above all, to secure ef- 
fects in expression hitherto obtained with difliculty on account 
t)f the longer exposure required. As regards the pictorial ef- 
fect capable of being obtained out of doors, the editor of this 
manual has obtained from the deck of a steamer swiftly mov- 
ing in one direction sharply defined and fully exposed nega- 
tives of steamers, yachts, and other ships, rapidly proceeding 
in an opposite direction. This exalted sensitiveness in dry 
plates is owing to the substitution of gelatine for collodion as 
the agent for presenting the sensitive bromide of silver in a 
pellicular form. 

While it is not necessarj^ that any detailed account of the 
gelatine emulsion process from its first publication by Dr. 
Maddox up to the present time be given, yet a few remarks- 



G-ELATINE EMULSION PEOCESS, 303 

of a somewhat historical nature will conduce to the better un- 
derstanding of the advances that have been made. 

As first presented to the public, the gelatine emulsion pro- 
cess was slow, in the sense of requiring a long exposure in the 
camera ; but after two years, others entered the field of inves- 
tigation, and succeeded in obtaining a degree of sensitiveness 
at first equal to, and subsequently exceeding, wet collodion. 
The method by which this sensitiveness was obtained was kept 
a secret. So possessed had the photographic mind been with the 
idea of the impossibility of any dry plate being more sensitive 
than the standard wet collodion that one professional manufac- 
turer of gelatine plates (Mr. R. Kennett) was compelled to issue 
them of a lower degree of sensitiveness than he had at first manu- 
factured, simply because photographers, not realizing the pos- 
sibility of such great sensitiveness, spoiled many plates from 
over-exposing. It was only after the publication of an article 
by Mr. Charles Bennett, descriptive of his method of prepar- 
ing the emulsion, by which he had secured certain exceedingly 
rapid effects not previously deemed possible, that public atten- 
tion became awakened to the great value of the new process 
for securing, in a manner more thoroughly than was pi^eviously 
possible, complete delineations of life and character. 

The discovery, although important, was in itself character- 
ized by great simplicity. Whereas hitherto the emulson had 
been prepared with as little delay as was compatible with al- 
lowing the chemicals employed to react upon each other, it 
was found that in proportion to the time allowed for such re- 
action to take place, so was the gain in sensitiveness within 
certain limits. 

A further stage in this discovery was reached when it be- 
came known that the protracted action of several days' prepar- 
ation of the emulsion might be condensed into that of a few 
hours by merely increasing the heat considerably beyond that 
at which liquefaction of the gelatine was effected, and that, 
consequently, by varying the intensity of the heat and dura- 
tion of exposure to it, either a quick or a slow working emul- 
sion could be prepared. 

The foregoing is believed to be all that is necessary to be 
given here of a historical nature ; minor details will be noticed 
as w^e proceed, with practical directions for preparing and 
using the emulsion. 

The first thing requisite is a suitable gelatine. Nelson's 
gelatine has acquired a deservedly high reputation in this con- 
nection, but in the hands of some it is found to be somewhat 



304 PRACTICE OF PHOTOGEAPHY. 

soft and apt to pucker up during subsequent stages of the 
manipulation, a tendency which is overcome by the admixture 
with it of a harder sample, that of the French brand, 
" Coignet's," being much preferred for the purpose ; but we 
have known a kind of gelatine which was quite unfitted for 
emulsion work, owing to its softness and solubility, having 
been rendered quite good by the admixture with it of chrome 
alum, an addition which, for this purpose, requires to be made 
with extreme caution and by very minute quantities, otherwise 
the gelatine will not only be rendered hard, but quite in- 
soluble. This is of less importance when all the emulsion 
made at a time is to be used in the coating of plates without 
allowing it to become cold. An excess is discoverable by per- 
mitting a small portion to congeal or solidify, and then 
noticing whether or not it has become insoluble. It is no 
secret that several manufacturers of plates add alum pretty 
freely to their emulsion in order to ensure a hard film and one 
which will not pucker up. 

Allusion has been made to the "instantaneous" photo- 
graphing of steamers and ships in motion from the deck of 
another steamer under full way. A detailed account of the 
method by which the emulsion was made, and the plates pre- 
pared for that occasion, will form a useful, practical lesson at 
this juncture. It is proper to state that, although these plates 
are much more sensitive than wet collodion, they are not the 
most sensitive that can be prepared. As regards certainty 
and reliability, however, they are altogether unsurpassed. Into 
a quart beaker place : 

Bromide of ammonium, 200 grains. 

Nelson's No. 1 gelatine, 80 " 

Water, 3 ounces, 

and allowed to stand for a quarter of an hour, by which time 
the gelatine will be swollen from the absorption of the water. 
It is well to dissolve the bromide previous to adding the gela- 
tine. At the end of the time specified, place the beaker in a 
saucepan of boiling water, and as soon as the gelatine is lique- 
fied, which will be the case as soon as it becomes wai-m, the 
saucepan is removed into a dark room, and the remainder of 
the manipulations performed by ruby light. 

A solution of nitrate of silver must, in the meantime, have 
been prepared of the following strength : 

Nitrate of silver, 330 grains, 

Water. . 3 ounces, 



ATINE EMULStuN PROCESS. 305 



and of this about half an ounce is poured into a graduate and 
dihited with half an ounce of distilled M'ater, after which it is 
added to the gelatine solution and well mixed with it. 

Tiiere are various appliances by which the admixture may be 
thoroughly made, and much ingenuity has been expended in 
devising such appliances, but one of the best consists in 
having a piece of a tolerably coarse and long-toothed vulcanite 
comb, of a length somewhat less than the diameter of the 
beaker, attached to a round wooden handle ten or twelve 
inches in length, much in the fashion of a miniature garden- 
rake, with this difference, that the teeth will be standing in the 
same direction as the handle. This is placed in the gelatine 
solution, and by rolling the handle between both hands a twirl- 
ing motion is imparted by which the silver solution is rapidly 
incorporated with the gelatine. After this addition allow the 
beaker to remain undisturbed for about a minute, then add the 
remainder of the silvei- in four or five doses, without diluting 
it as in the case of the first silver added,' and with a vigorous 
application of the agitator after each addition. 

The saucepan having been placed upon a gas stove in the 
dark room, the gelatine emulsion is allowed to remain under 
the influence of the high temperature for three quarters of an 
hour. This time may be shortened, and a good emulsion be 
still eventually obtained, but it will be much less sensitive than 
if allowed to remain for the time mentioned. 

By lowering the temperature of the water in the saucepan 
from the boiling point recommended to 150 deg. Fahr,, and 
greatly prolonging the time during which emulsilication is al- 
lowed to take place, an equal degree of sensitiveness will be 
obtained. It may here be observed that several manufacturers 
prefer a moderately prolonged " cooking " at a medium tem- 
perature, to a quicker one in boiling water. 

Seeing that the ultimate sensitiveness and quality are so 
greatly dependent upon the duration of the exposure to the 
heat, it is of great consequence to manufacturers and others 
who wish to prepare plates which shall at all times maintain a 
reputation for equality and uniformity of sensitiveness to note 
carefully the time of " cooking " the emulsion to secure 
the best effects, and then to adhere strictly to it ever after- 
wards. 

The beaker with its contents having been removed from the 
hot vi^ater, the emulsion is allowed to cool down to 100 deg. 
Fahr., or a little over that temperature. There is then added 
to it a plain solutionof gelatine which has been prepared during 



306 PRACTICE OF PHOTOGRAPHY. 

the time the afore-described operation lias been going on. It is 
composed of : 

French (coignet) gelatine, ^ ounce. 

E'elson's, i " 

Water, 6 ounces. 

This is liquefied by first soaking together and then applying 
heat in the manner already described. JN'o more heat is applied 
than suffices to cause its perfect liquefaction. The addition of 
this gelatine solution to the emulsion, neither of them being 
much above 100 deg. Fahr., is followed by an application of 
the agitator to ensure perfect admixtuie. 

The beaker is now, with its contents, placed in a perfectly 
light-proof box or case, and allowed to stand for several hours, 
or until the emulsion has become a cold firm jelly, which con- 
sists of gelatine, bromide of silver, and nitrate of ammonia. 
To eliminate the last of these, the jelly must be washed in cold 
water, into which a few small pieces of ice may advantageously 
be thrown to ensure a low temperature. The most con- 
venient way by which to wash the emulsion is to place it, 23re- 
viously cut into pieces of convenient dimensions, into a thin 
porous canvas cloth, sixteen or eighteen inches squai'e, then 
twist the neck and immerse it in the cold water. So much pres- 
sure must be applied as to cause the emulsion to be forced out 
through the meshes of the canvas, by which it is broken up so 
completely as to enable the water to dissolve all the soluble 
nitrate of ammonium, which is effected partly by the direct 
action of the water u]3on the now large surface exposed, and 
partly by a species of dialysis in which the shreads of jelly form 
the septum for the separation of the soluble salts contained in 
their substance. After a quarter of an hour the water is 
poured off, and the jelly again forced through the canvas into 
fresh water, in which it may be allowed to remain for half an 
hour. A third washing, under similar circumstances, will 
prove sufficient. The emulsion is next freed from the water 
as completely as possible by draining, and it is then gathered to- 
gether in the beaker, subjected to as low a degree of heat as 
suffices to liquefy it, and an ounce of alcohol then added, to- 
gether with as much warm water as will make up the volume 
to twenty ounces. After being stirred and filtered through 
muslin, the preparation of the emulsion is completed, and it is 
now ready for use in coating plates. 

As above described, the emulsion made will yield plates of 
a very satisfactory nature, both as regards sensitiveness and 



GELATINE EMULSION PROCESS. 307 

quality of mixture, but numerous deviations may be made 
from it in the apparatus employed, the methods of operating, 
the proportions of the ingredients employed, and in the sub- 
stitution of others, and some of these we shall now refer to. 

We have alluded to certain points of distinction between 
the Nelson and the Coignet gelatines. It has been found to 
be a characteristic of the former that it has a slightly alkaline 
initial reaction, which becomes more pronounced after being 
kept at a temperature of about 100 deg. Fahr., for a few days. 
It is also found to lose to a considerable extent its power of 
setting by such treatment. It is possible for gelatine to lose 
this power of setting without any decomposition having set in, 
but there is no doubt that prolonged "cooking" at a high tem- 
perature induces putrefactive decomposition. By this cook- 
ing the first change that takes place is the splitting up of 
gelatine into two substances, semi-glutin, insoluble in alcohol, 
and precipitated by platinic chloride ; and hemi-coline, which 
is soluble in alcohol and not affected by platinic chloride 
(Eder.) This splitting up is diflierent from putrefaction. 

A very high degree of sensitiveness is obtained when by an 
exposure to a pretty smart heat, as described in the directions 
above given for preparing emulsion, a slight decomposition 
has set up. This, while favoring sensitiveness, is hostile to 
setting or jellifying, hence the necessity for adding to the 
" cooked " gelatine emulsion a large proportion of gelatine, 
which has not, by prolonged exposure to heat, lost this prop- 
erty. This shows in wdiat manner the sensitiveness obtained 
by long cooking of several days' duration has its equivalent in 
an exposure to a higher temperature for a brief period. 

Knowing that ammonia is formed as the product of that 
prolonged heating which induces sensitiveness, it has been pro- 
posed to add ammonia to the emulsion, both in a direct man- 
ner, and also indirectly by means of ammonio-nitrate of silver, 
which is still an oxide of silver dissolved in nitrate of ammonia, 
and in this way to secure a higher degree of sensitiveness. 
The results hitherto obtained by this modification have not yet 
been such as to lead to a desire to abandon the simpler method. 
Nothing has yet surpassed the system of emulsifying with a 
soft and soluble gelatine, one which, if not alkaline at the 
starting, becomes so after being in solution for a brief period, 
passing into a state in which the setting point is lowered several 
degrees after the slight boil to which it has been subjected, 
and then making an addition of a hard sample which is more 
slowly absorbent of water, and sets at a tolerably high tem- 
perature. 



308 PRACTICE OF PHOTOGEAPHT. 

There are some kinds of gelatine whicli have not in process 
of manufacture been rendered altogether free from fatty mat- 
ter, by which defects are alleged to be caused in the negatives. 
Such defects usually disappear by the addition of ammonia to 
the emulsion, by which a saponaceous compound is formed 
that does not appear to be detrimental to the general working. 

An exceedingly hard gelatine is not favorable to the ob- 
taining of a high degree of sensitiveness, that is when the 
emulsiiication has been effected with such a kind. It is less 
amenable to the decomposing action of the heat during the 
emulsiiication. Hard, and by comparison non-porous, gela- 
tine approximates more nearly to a collodion film, which has 
not yet been found to compare with gelatine in sensitiveness, 
unless when, under certain conditions described in another 
place, an addition of gelatine has been made to it in presence 
of a solvent common to both pyroxiline and gelatine. 

We shall now describe a modification of the method already 
described in which the bromide of ammonium and plain 
nitrate of silver are supplanted by bromide of potassium and 
ammonio-nitrate of silver. The sensitiveness of the emulsion 
thus obtained is several times in excess of that secured by wet 
collodion. 

In a beaker place : 

Bromide of potassium, 370 grains. 

Gelatine, ■ . 615 " 

Water, 10 ounces. 

The precise kind of gelatine is here immaterial, but it is not 
advisable that it be alkaline. After allowing it to soak for 
some time, as in the previous formula, liquefy by placing the 
beaker in water of 105 deg. Fahr. Now add by little at a 
time, and shaking or stirring after each addition, the follow- 
ing : 

Nitrate of silver, 462 grains, 

Water, 10 ounces, 

to which has been added (after solution) enough ammonia, 
drop by drop, to just barely dissolve the precipitate which is 
at first thrown down by the addition of the ammonia. When 
the liquid is once more clear, pour in, as ]ust directed, and 
allow the emulsion to cool down to 95 deg. Fahr. The silver 
should be rinsed out of the vessel in which it was dissolved 
by fourteen drams of water which is then added to the emul- 



GELATINE EMULSION PEOCESS. 309 

sion. During the foregoing operation the emnlsion may have 
been removed from the water bath, the vessel containing it is 
once more immersed and the temperature raised to 95 deg. 
Fahr., and then allowed to cool down gradually to the tem- 
perature of the room, after which the emnlsion, subject to fil- 
tration, is ready for use. 

Concerning the washing of the jellified emulsion to remove 
from it the crystallizable salts formed by decomposition dur- 
ing the preparation, there are four ways by which this may be 
effected, viz., by precipitation of the gelatine emulsion by 
alcohol ; by the breaking up of the emulsion into small fila- 
ments in the manner already described, and which we prefer to 
any other ; by dialysis, and by a more simple washing, which 
will be first described. 

In this, the emulsion is poured out into a large flat dish to 
the depth of about a quarter of an inch. It is then allowed 
to become cold and firm, when it is broken away from the 
sides and bottom of the dish by means of a bone or horn spa- 
tula, a common paper-knife being much used for the purpose. 
After having been subjected to several changes of water dur- 
ing a period extending over a few hours, the emulsion is finally 
drained and liquefied by heat. By this treatment all the solu- 
ble salts will have been removed. 

When dialysis is employed to effect the same purpose, the 
best form of dialyser consists of a porcelain or glass ring of 
about four inches in depth and of a diameter to suit the quan- 
tity to be operated upon. One that we have employed suc- 
cessfully is six inches in diameter. For small quantities, a 
porcelain jelly pot having its bottom knocked out makes a 
good extemporized dialyser. Over the bottom is stretched 
vegetable parchment as tightly as possible and retained in its 
place by either a piece of string or a rubber band. A large 
vessel containing warm water, about 105 deg. Fahr., having 
been got ready, the dialyser is placed in it and supported in 
such a manner that the liquid emulsion when poured into it 
shall have its surface on a level with that of the water out- 
side, and that a depth of several inches of water shall be below 
its bottom. When covered up and allowed to stand in re- 
pose, by a process of osmose all the crystallizable salt present 
in the emulsion passes out through the parchment septum into 
the water, which ought to be changed two or three times dur- 
ing the process. There is a very simple method by which it 
may be ascertained whether the salts have been removed ; if 
a little of the emulsion be at first spread upon a plate of glass 



310 PEACTICE OF PHOTOGKAPHY. 

the presence of the crj^stalline salt will be apparent upon dry- 
ing it ; by making a second trial after half an hour its pres- 
ence will be much less marked ; a further trial at a more ad- 
vanced stage showing a total absence of the salt. 

In preparing plates for coating, the question of a substratum 
requires a little consideration. Some kinds of gelatine give a 
film which has a strong tendency to pucker up and leave the 
glass during subsequent treatment of the negative. The ten- 
dency to move may be destroyed by coating the plate with a 
substratum. One of the best of these is found in an exceed- 
ingly thin film of insohible gelatine, which is easily prepared 
and readily applied. Dissolve 120 grains of any good gela- 
tine in a pint of water, and add to it 5 grains or chrome alum 
previously dissolved in a little water. This may be applied by 
means of a clean fine sponge. It leaves a thin and almost im- 
perceptible film, which effects a strong union between the 
glass and the emulsion subsequently applied. 

Highly diluted albumen, about the white of an egg to a pint 
of water, is also used for the same purpose by many. One 
would at first sight imagine that the attenuated stratum of 
albumen, being soluble in water, would be removed by the 
gelatine emulsion in which water is the solvent, but the expe- 
rience of many is given in attestation of its serving in a satis- 
factory manner the purpose for which it was applied. Pow- 
dered steatite, or French chalk, which proves so useful in 
causing the adhesion of a collodion film under similar circum- 
stances, is also used by some in connection with gelatine. 

Plates are most easily coated by means of a deep spoon, 
which, when dipped into the vessel of emulsion, removes just 
so much as suffices for a plate of any determined size. The 
nature or form of the spoon may be conveyed by that of a 
soup ladle, or a tobacco pipe of clay or wood, the capacity be- 
ing such as to hold emulsion enough for the area that is to be 
covered. By its agency, and keeping the emulsion at a uni- 
form temperature during the operation, a few gross of plates 
may be prepared with absolute uniformity. It is convenient 
to have one such vessel or spoon for each size of plate that is 
coated, with the dimensions of the plate written on the han- 
dle, such as 22 x 17, 12 x 10, 5 x 4, and so forth. By the exer- 
cise of dexterity, aided by a pneumatic plate holder and a 
chisel-shaped piece of thin bone, vulcanite or any substance 
that may be made thin and flexible, the emulsion may be 
evenly and rapidly spread over the surface of the plate, which 
is then removed by an assistant and placed upon a board or 



GELATINE EMULSION PKOCESS. 311 

slab previously leveled with care. If the weather is moder- 
ately CO0I the eniiilsion sets rapidly, after which, but not until 
then, the plates may be either reared up or placed in a drying 
box in any position irrespective of level. If the manufacture 
of plates is to be conducted on a large scale, each maker will 
establish a routine system adapted to his own requirements 
and surroundings. It is well to bear in mind that after a gela- 
tine film has set it does not again liquefy unless it be exposed 
very soon to a temperature considerably higher than that at 
which the setting took place. As coldness favors rapid set- 
ting, expedients may be had recourse to during hot weather to 
lower, by means of crushed ice and salt, the temperature of 
the atmosphere surrounding the setting slab upon which the 
plate is placed after being coated. 

The thorough drying of the plates may be effected either 
spontaneously in a dark room, or in a box specially constructed 
for the purpose. The principle upon which such a box is con- 
structed is such as to allow of a current of pure air to pass 
over the surface of each, by which it is cliarged with moisture 
and passes off. • The box must be tight, so as not to admit the 
faintest ray of light, and the shelves upon which the plates 
are placed must be so arranged as to allow the air, which is 
admitted at the bottom, to pass upward in a zigzag manner, 
so as to expose the surface of each to the ascending draught. 
This current is caused by placing a lighted lamp of any con- 
struction requiring a large supply of air upon the top of the 
drying box, and contriving it in such a manner that no air can 
be supplied to the burner except what is fed to it through an 
aperature in the top of the box ; hence, by igniting the lamp, 
an upward current of air is immediately started through the 
box and over the surface of the plates. Some have mistakenly 
reversed this order and placed the lamp below, allowing a cur- 
rent of air, heated and laden with the gaseous products of 
combustion, to have access to the plates. This is altogether 
wrong. 

The various manufacturers pack their dry gelatine plates in 
various ways. Owing to the hardness of the film when dry, 
the same precautions against accident which are necessitated 
when working with collodion plates are not here required. 
Some pack them face to back, others face to face without any- 
thing between them, others interpose between them a mar- 
ginal slip of paper or thin card ; but a way which we prefer 
to any is to have provided a supply of tissue paper cut in 
sheets the same dimensions as the plates, and the packing 



312 PRACTICE OF PHOTOGKAPHY. 

then done in the following order : First, a plate laid down 
face upward ; next, a sheet of paper ; then all the rest of the 
plates that are to form the package laid face down with a 
sheet of paper between each, with a primary wrapping of 
tissue paper followed by one of opaque paper. Plates so put 
up may be relied upon to remain quite good for a long time — 
certainly for several years. 

Allusion has been made to the photographing of a steamer 
in motion. This, however, is a feat which falls short of taking 
a perfect picture of the " Derby " race at the moment the 
horses are within a few feet of the winning-post, and are, 
therefore, at their utmost speed. Such a picture of the 
" Derby " of 1882 was taken by Mr. A. L. Henderson ; and, 
as this gentleman's methods differ somewhat from those of 
others, and of what we have described, he has kindly fur- 
nished the following details — never previously described in 
full— for this " Manual." 

The gelatine is neutral and not too soft, and he prefers 
emulsifying with a very small quantity of it. 

Gelatine, 30 grains. 

Bromide of potassium, 150 " 

Iodide of potassium, 2 '' 

Water, 1 ounce. 

Alcohol, > 2 ounces. ■ 

Ammonia, (.880), 3 drams. 

These are placed in a jar holding from 2 to 3 pints. The 
bromide, iodide, and gelatine are first dissolved in the water, 
after which the ammonia and alcohol, having been previously 
mixed, are added. 

The heat having been raised to from 120 deg. to 140 deg. 
Fahr., an addition is made of : 

Nitrate of silver, . . . 200 grains. 

Water, 2^ ounces. 

This is stirred in, after having been heated to the temperature 
of the other, in a fine stream. 

After being allowed to stand for 5 minutes, the bulk of 
the gelatine required (240 grains) is then added, dry, without 
any previous soaking in water. We may here premise that 
the gelatine we saw used by Mr. Henderson is in very thin 
sheets. The heat remaining in the solution is quite sufficient 
to dissolve and properly emulsify the gelatine last added. 



GELATINE EMULSION PEOCESS. 313 

The emulsion thus made is washed either in the usual wa}^, 
or by precipitation with alcohol, the latter being preferred. 
To this end, li ounces of alcohol, heated to a temperature of 
100 deg. Fahr,, are slowly added, stirring all the time. The 
emulsion, if properly prepared, will be in a ilocculent state, 
neither precipitating nor adhering to the stirring rod or sides 
of the vessel at iirst. It is now allowed to cool, by which 
time the emulsion will have become a cake at the bottom of 
the vesseh The liquid is now poured oif, and the emulsion is 
removed from the bottom by means of a horn spoon, and 
placed under a small stream of M^ater, which is allowed to 
trickle through it fur a few hours. After this it is liquefied, 
and water enough added to make 8 or 10 ounces. 

Those pictures we saw made were developed by the fol- 
lowing : 

:N'o. 1 (Stock). 

Pyrogallic acid, 100 grains. 

Citric acid, 5 " 

Water, 10 ounces. 

The citric acid is dissolved previous to the addition of the 
pyrogallic. 

No. 2 (Stock). 

Bromide of potassium, 2 drams, 

Ammonia (.880), 1 ounce. 

Water, 2 ounces. 

No. 3. 

Water, .1 pint. 

OfJNo. 2, 21 drams. 

To develop : Take of No. 1, 1 dram, and mix with 7 drams 
of No. 3, employing a dish for developing. 

If, under any circumstances, intensification be desired, a 
saturated solution of bichloride of mercury, followed by diluted 
lime water, is recommended. 

The variety of developers employed to suit various kinds of 
plates, or to gratify individual tastes, are such as would fill 
many pages of this " Manual," if here given. In the formula 
of Mr. Henderson, just given, citric acid is present with 
the pyrogallic in the stock solution to prevent oxidation. Sul- 
phite of soda is also extensively employed for this purpose. 



814 PEACTICE OF PHOTOGEAPHY. 

Mr. Berkeley, by wliom it was first recommended, gives the 
following as a suitable proportion in wliich to use it : In one 
ounce of water dissolve 200 grains of tlie sulphite ; then neu- 
tralize by an addition of a strong solution of citric acid, about 
4 grains of the acid sufficing ; next add 50 grains of pyrogallic 
acid. This solution is of a ten-per-cent. degree of strength, 
and will remain colorless for months. Mr. Wilkinson makes 
his developer in a form which he believes to possess an ad- 
vantage over that just given : 

Xo. 1. 

Sulphite of soda, 4 ounces. 

Water, .40 " 

Dissolve ; then add sufficient of a saturated solution of cit- 
ric acid to produce a slight acid reaction upon litmus j)aper. 
'Next add 1 ounce of pyrogallic acid, and make up the bulk 
to 54 ounces. Each ounce of this solution contains 8 grains of 
pyro. 

No. 2. 

Ammonia, .880, 1 ounce. 

Bromide of potassium, 180 grains. 

Water, 40 ounces. 

Equal parts of Nos. 1 and 2, mixed together, give a 4 
grain solution suitable for developing. 

In the developer of Mr. B. J. Edwards, glycerine is intro- 
duced with advantage. 

Make two stock solutions, as follows : 

No. 1. 

Pyrogallic acid, 1 ounce. 

Glycerine, 1 " 

' Alcohol, 6 ounces. 

Mix the glycerine and the alcohol, and add to the pyrogallic 
acid. 

No. 2. 

Bromide of potassium, 60 grains. 

Liquor ammonia, . 1 ounce. 

Glycerine, 1 " 

Water, 6 ounces. 



FEEEOUS OXALATE DEVELOPMENT. > 315 

These stock solutions will keep a long time. To make the 
developer, acid one part of No. 1 to fifteen parts of water, and 
label this bottle D (developer). In another bottle mix one 
ounce of JNo. 2, with fifteen ounces of water, and lable it A 
(accelerator). Either of these latter solutions will keep two 
or three days. When required for use, pour into a clean glass 
measure equal parts of D and A, adding the A just before 
using. Place the plate in a flat tray, and pour the developer 
over the surface, avoiding air bubbles. The image appears 
in a few seconds, and rapidly acquires strength. 

Ferrous Oxalate Develojpment. 

Make a saturated solution of neutral oxalate of potash, and 
add a few crystals of oxalic acid, to ensure its being in a 
slightly acid condition, together with 20 grains of bromide of 
potassium for each pint of solution. 

In another bottle make a saturated solution of protosulphate 
of iron, and add sulphuric acid in the proportion of about ten 
drops per pint. 

When about to use it, mix one part of the iron solution 
with six parts of the potassic oxalate solution, and pour into a 
tray. The image appears slowly, and soon becomes very 
intense. 

If greater rapidity in the development be desired, the pro- 
portions of the two mixtures may be varied ; one of the iron to 
four of the oxalate being employed. 

Fixing is effected by a nearly saturated solution of hypo- 
sulphite of soda. 

Prolonged washing is requisite in order to get the fixing 
solution removed from the film. 

Immersing the negative in a saturated solution of common 
alum is beneficial; it hardens the film, and renders it less 
liable to frill, which sometimes takes place in very hot 
weather. This alum bath may be used after developing and 
before fixing. 

Intensifying. — The plate should be washed and dried pre- 
vious to its being intensified, should intensification be found 
necessary. The following modification of one of the oldest 
collodion intensifiers is much employed : 

ITo. 1. 

Bichloride of mercury, 60 grains. 

Water, 6 ounces. 



316 PKACriCE OF PHOTOGRAPHY. 

Ko. 2. 

Iodide of potassium, 90 grains. 

Water, 2 ounces. 

No. 3. 

Hyposulphite of soda, 120 grains. 

Water, ^ 2 ounces. 

'No. 2 is poured into No. 1, and tlien No. 3 is added. The 
plate is immersed into the solution, which quickly imparts- 
great density. 




CHAPTER XIX. 



THE COLLODION EMULSION PKOCESS. 

Instead of going into the numerous ramifications con- 
nected with collodion emulsions of various kinds, we shall here 
confine our remarks to the practical details of one good and 
thoroughly reliable process, the best of all that we have tried. 
It is essentially that of the late Mr. Henry Cooper. Mix to- 
gether — 

Bromide of zinc, 80 grains. 

Lactate of ammonia, 20 minims. 

Plain collodion, 5 ounces. 

The lactate of ammonia is syrupy, and is prepared by neut- 
ralizing lactic acid with ammonia. 

Dissolve in a test tube 150 grains of nitrate of silver in about 
a dram of water by the aid of heat, and add three ounces of 
alcohol. Raise the heat to near the boiling-point, and pour the 
solution slowly and with agitation into the collodion (in the 
dark). Keep in a warm place, and shake up at frequent inter- 
vals. After a day add twenty drops of nitric acid, shake well, 
and pour out into a fiat dish to set. When nearly solid break 
up the thick film with a paper-knife and wash well with water, 
which must be changed several times. After drying, dissolve 
the pellicle in a mixture of five ounces of ether and five 
ounces of strong alcohol. 

This emulsion keeps well, and produces images of a very 
high class. The plates are coated, dried, and then stored away 
ready for exposure. 

A useful developer for a collodion emulsion plate is com- 
posed by — 

» j Pyrogallic acid, ....... 3 grains. 

I Water, L ounce. 

p i Bromide of potassium 10 grains. 

I Water, 1 ounce. 

p ) Carbonate of ammonia 30 grains. 

\ Water, 1 ounce. 



318 PRACTICE OF PHOTOGRAPHY. 

To every half-ounce of A add two drops of B and three 
drops of C. 

The negative is fixed with either cyanide of potassium or 
hyposulphite of soda. 

Dry Bath Plates. — Dry collodion plates may be prepared 
by the bath by sensitizing as usual and then thoroughly wash- 
ing off the nitrate of silver. A preservative is then applied, 
which may consist of one or other of the following: — A fifteen- 
grain solution of tannin ; a strong decoction of coffee ; a solu- 
tion of gum ; a wash of pyrogallic acid with or without gum ; 
or albumen diluted with an equal part of water. If the last be 
used the plate should be immersed in hot water for a few sec- 
onds to coagulate the albumen. All these preservatives yield 
plates less sensitive than those prepared by the emulsion de- 
scribed above. 




CHAPTER XX. 



WILLIS PKOCESS FOR ENGINEERS PLANS. 

This process is different from other methods, inasmuch as it 
produces a positive from a positive, and vice versa. Hence, it 
is eminently adapted for the reproduction of engineers' plans 
and drawings without the aid of a camera. It is based on the 
fact, long ago ascertained, that aniline with chromic acid 
strikes a deep blue color. 

Paper. — Select a thick, smooth- surfaced, and highly-sized 
plain paper. The thickest quality of Saxe will be found about 
the best that can at present be obtained, but it can still better 
be adapted for this purpose by hot-pressing each sheet before 
applying the chemicals. The object of hot-pressing is to com- 
press the fibre into the most compact state, and thus to keep 
the chemicals as much as possible on the surface. 

Sensitizing Solution. 

Bichromate of ammonia, ... 50 grains, or less. 
Phosphoric acid (solution), . . .1 dram (fluid). 
Water, 1 ounce. 

The phosphoric acid to be used in the above formula is the 
tribasic form, not readily obtained at an ordinary chemist's 
shop, except in a very dilute form, and of uncertain strength. 
The following indications will guide the operator as to the 
strength or quantity of the phosphoric acid. If too little acid is 
present the picture willdevelop of a reddish tint ; if too much, 
the color will be green. The best tint is a purplish-black, ob- 
tained by regulating the acids according to the above indica- 
tions of color. Yet, after all, one need not be very par- 
ticular on this point, because the color can be afterwards 
changed. 

To sensitize the paper, it is pinned down by the corners to a ' 
drawing-board, and the above solution is spread over its upper 
surface evenly with a broad camel's-hair brush, or otherwise, 
and dried quickly by the fire, or in a warm room. The object 
of drying quickly is to prevent the solution from penetrating 



320 PRACTICE OF PHOTOGRAPHY. 

far into the paper. The color of the dried surface should be 
of a deep and uniform orange tint. 

The sensitive papers should, if possible, be used during the 
same day on which thej are prepared. If kept for twenty- 
four hours or upwards, a considerable diminution of sensitive- 
ness is a])parent, and often they will not take the aniline vapor 
uniformly. 

The exposure varies according to the intensity of the light, 
and the resisting medium of paper, etc., through which the 
light has to be filtered. The transparent or translucent picture 
is laid on the glass of the printing-frame, and the surface of 
the sensitized paper is pressed in contact, just as in the ordinary 
printing process. The proper time of exposure, is not, how- 
ever, so readily detected by the eye as in silver prints ; but a 
very short experience will render the matter easy. When ex- 
posed for the proper time, a well-defined outline of all the 
dark portions of the print should be apparent in orange color 
on a darker ground. Generally speaking, the time of exposure 
is about one-fifth of that required for the same subject on the 
most sensitive silver-chlorized paper. But the indications of 
over and under-exposure are easily discerned when we come 
to the 

Development, — A shallow wooden box (about two inches 
deep) with a lid of the same material, f(irms a very convenient 
developing dish. Pin to the inner surface of the lid two or 
three folds of bibulous papei*. Moisten them with a solution 
composed of common commercial aniline (four drains) and 
benzole (two ounces or more). About a dram of this mixture 
will be suflicient to develop a print about two feet square. 
Place inside, on the bottom of the box, as many prints as 
will lie thereon, but not on the top of each other. Put on the 
lid. In about twenty minutes, more or less, the whole should 
be fully developed by the vapor of aniline falling upon them. 
]S^ow is the time to examine for over or under-exposure. If 
the exposure has been a little too long a feeble green, blue, 
or reddish image (the color depending to a great extent on the 
amount of phosphoric acid in the bichromate) will be percep- 
tible. But if the time of exposure has been greatly exceeded, 
no image at all, or at least a very feeble one, will be apparent, 
because the light has had time to penetrate through the dark 
parts of the intercepting medium and decompose the bichro- 
mate. 

Tiie symptoms of under-exposure are just the reverse. In 
such cases the exposed papers develop all over, so to speak, the" 



WILLIS' PKOCESS FOE ENGINEEKS' PLANS. 321 

high lights being discolored by the aniline vapor nearly as 
readily as the shadows. 

The development being complete, it is of little consequence 
whether the resulting image be red, green, or blue. The tone 
■can be changed in the simplest manner. But before doing so 
it is advisable to fix the picture by washing off the soluble 
salts. This is done in common water. Immerse the print in 
water acidulated with nitric or sulphuric acid ; quickly the 
original color, whatever it may have been, will change to a deep 
bluish green. Wash the print and again immerse it in water 
containing a few dro])s of ammonia, when, almost instantly, the 
picture turns a rosy-purple shade. Try the acid again. The 
green will be deeper than before. Wash once more, and im- 
merse in a weak solution of ferrocyanide of potassium ; then 
the green will be of the most delicate spring-leaf kind. These 
chameleon-like changes of color may be made over and over 
again ; but the most remarkable fact connected with them is, 
that after every change the colors are much improved, and a 
greater general vigor is impai'ted to the image. 




CHAPTEE XXI. 



MICEO-PHOTOGEAPHY. 



Select a window facing south or south-west, throw it open, 
place a firm table near to it, fix the microscope at one end of a 
4 or 5 feet stout deal clamped board resting on the table, and 
so arrange the instrument that, when turned horizontally, the 
sun's image may be thrown by the mirror or prism, set at aright 
angle to the path of the rays, through the axis of the instru- 
ment, the objective and eye-piece being removed. Select the 
camera to be used, and so place it on the board that the eye- 
piece end of the microscope may be passed through the centre 
of a stout cardboard cap, or brass tube lined with cloth, made 
to take the place of the ordinary brass cap of the camera com- 
bination, the cells carrying the lenses having been unscrewed. 
Receive the image of the sun on the ground-glass screen, ob- 
serving that the spot of light occupies its centre, firstly throw- 
ing the ordinary focusing cloth over the end of the camera and 
microscope, so as to totally exclude any light admitted into the 
camera by the side of the tube of the microscope. Now attach 
a low power objective to the microscope, and note if the sun's 
image remains central on the focusing screen ; if not, adjust 
the mirror, camera, and microscope until this be determined, 
then mark the position of the camera and microscope on the 
deal board, and clamp them there. Select the object to be photo- 
graphed, place it on the stage, as usual (it is better generally to 
narrow the central aperture of the stage to nearly the size of 
the object by a pierced card blackened or black paper gummed 
to the under side of the slide), and shut up the camera so that 
with the hand on the coarse rack adjustment it may be focus- 
ed at the same time as the head is inclined towards the focus- 
ing glass, so that with the focusing cloth thrown over it, the 
eye may note the definition, size of the image, and equality of 
the illumination. Should it be determined to use the full 
range of the camera, some arrangement for working the rack 
movement will be necessary, such as a rod attached at its oppo- 
site end by a lever or grooved wheel, connected by a band to 
the mill head of the coai-se or fine rackwork. Suppose the sen- 



MICEO-PHOTOGEAPHY. 323 

sitive plate prepared and ready to be dropped witli its carrier 
into the camera, note that the ilkimination is perfect, then shut 
off the light by any simple plan ; insert the carrier, draw up 
the shutter, and ap|)ortion the period of exposure to the color 
of the object, the distance of the screen, the brightness of the 
light, and condition of the chemicals or sensitiveness of the 
plate ; this may range from one to several seconds. The nega- 
tive, when developed and cleared, may show a very imperfect 
or ill-defined image from the want of coincidence of the visual 
and actinic foci of the objective, which for that distance may 
be ascertained by tentative trials, withdrawing the objective 
from the object by a turn or part of a turn of the fine adjust- 
able milled head, or what is perhaps preferable, by having a 
suitable thin convex, long focus, spectacle lens, ground down 
and fitted into a cell which screws into the place of the back 
stop of the objective. The focus of the lens required may be 
from 8 to 12 or 14 inches; or a large cell with parallel glass 
slides, containing a deeply-colored solution of ammonio-sul- 
phate of copper, may be placed between the mirror and the 
stage, to shut off the non-actinic rays. Some of the low pow- 
ers are found to have their chemical and visual foci so nearly 
coincident that they require no extra adaptation. The actual 
position of the objective for a given distance once found, a 
mark may be made on the board at which the sensitive plate 
was placed, to save further trouble. 

AVith medium powers more care is required, and the adjust- 
ment collar for the thickness of the cover must be attended to ; 
what is needed in the objective being rather under correction 
for color, though correct for spherical aberration, which latter 
alters with the distance between the object and the image. 
Generally some form of condenser is necessary to concentrate 
the light from the plain mirror or prisim. These are of 
various kinds, and are as used in the ordinary conditions of the 
microscope, but it has been found best to employ lenses of 
some size, of course taking care that the focus is so arranged 
as not to scorch the object. In some cases there is a difiiculty 
in getting rid of the diffraction lines at the edges of objects, 
and then a cap containing a piece of ground glass in a cell, 
slipped over the top combination of the achromatic condenser, 
and so placed as to become a bright radiant of softened sun- 
light wherewith to illuminate the object, is found to have great 
advantages, though the time of exposure is increased. 

With high powers, the visual and actinic foci so nearly ap- 
proximate that they are often used as if really coincident ; but 



324 PEACTICE OF PHOTOaEAPHY. 

even here it will be found a better plan to insert the ammonio- 
sulphate of copper cell, or else to use a prism of some con- 
siderable dispersive power, and illuminate the object by the 
violet end of the spectrum. At the same time as the powers 
increase, it becomes very important to employ an examining 
glass, as a Ramsden's positive eye-piece, for the best point of 
the image as regards definition on the focusing screen, or even 
to substitute for the ground glass a polished piece of plate 
glass, Dr. Woodward's method, always remembering that the 
more enlarged the image, the less sharp will it appear ; more- 
over, it is objectionable to strain the lens to work beyond its 
suitability, or to use the eye-pice with it without especial 
care. 

If a heliostat be employed for this form of apparatus, it may 
be placed on the window sill, having taken the necessary pre- 
cautions for its position as regards the sun and its correct 
level ; or, it may be fixed to the side of the top of the front 
part of the camera. Much good work can be done without it 
if the chemicals be in good working order. With high power 
immersion lenses the time of exposure is lessened, and they 
have other advantages; but whatever means may be taken to 
illuminate the object, it is absolutely necessary the whole ap- 
paratus remains free of vibration. Indeed, after lifting the 
shutter of the camera, it is well to pause before removing the 
card from the stage or objective. 




CHAPTEE XXII. 
THE OPTICS OF PHOTOGRAPHY. 

ON THE NATURE AND PROPERTIES OF LIGHT. 

Although in a manual like this an elaborate disquisition on 
optics would be as much out of place as one on chemistry, yet 
it is considered desirable that such an account be given of the 
nature and properties of light, as applied to photographic 
lenses, together with an account of the construction and uses 
of such lenses, as shall enable the reader to acquire a clear 
idea of the most important of all the appliances involved in, 
the production of a picture. 

Section I. 
The Compound Nature of Light. 

The ideas entertained on the subject of light, before the 
time of Sir Isaac IN^ewton, were vague and unsatisfactory. It 
was shown by that eminent philosopher that a ray of sunlight 
was not homogeneous, as had been supposed,but consisted of 
several rays of vivid colors, united and intermingled. 

This fact may be demonstrated by throwing a pencil of sun- 
light obliquely upon one side of a prism, and receiving the 
oblong image so formed upon a white screen. 

The space illuminated and colored by a pencil of rays an- 
alyzed in this way is called " the solar spectrum," in which 
seven principal colors may be distinguished in the solar spec- 
trum, viz., red, orange, yellow, green, blue, indigo and violet. 
Sir David Brewster has made observations which lead him to 
suppose that the primary colors are in reality but three in 
number, viz., red, yellow and blue, and that the others are 
compound, being produced by two or more of these overlap- 
ping each other ; thus the red and yellow spaces intermingled 
constitute orange ; the yellow and blue spaces, green. 

The composition of white light from the seven prismatic 
colors may be roughly proved by painting them on the face of 



326 



OPTICS OF PHOTOGRAPHY. 



a wheel, and causing it to rotate rapidly ; tliis blends them to- 
gether, and a sort of grayish- white is the result. The white is 
imperfect, because the colors employed cannot possibly be 
obtained of the proper tints, or laid on in the exact propor- 
tions. 

The decomposition of light is effected in other ways besides 
that already given : 

First, by reflection from the surfaces of colored bodies. All 
substances throw off rays of light which impinge upon the re- 
tina of the eye and produce the phenomena of vision. Color 
is caused by a portion only, and not the whole, of the elemen- 
tary rays, being projected in this way. Surfaces termed white 
reflect all the rays ; colored surfaces absorb some and reflect 



Violet. 



IridiRO. 




Fig. 15. 

others ; thus red substances reflect only red rays, yellow sub- 
stances, yellow rays, etc., the ray which is reflected in all cases 
deciding the color of the substance. 

Secondly, light may be decomposed by transmission through 
media which are transparent to certain rays, but opaque to 
others. 

Ordinary transparent glass allows all the rays constituting 
white light to pass; but by the addition of certain metallic 
oxides to it whilst in a state of fusion, its properties are modi- 
fied, and it becomes colored. Glass stained by oxide of cobalt 
is permeable only to blue rays. Oxide of silver imparts a pure 
yellow tint ; oxide of gold or suboxide of copper, a ruby 
red, etc. 

PEOPEETIES OF LIGHT. 

Light possesses a threefold property, viz., heating, illumi- 
nating, and chemical action, now usually termed actinism. 
These properties have up to a recent period been associated with 



EEFEACTION OF LIGHT. 327 

certain rays ; for example, the yellow ray is popularly spoken 
of as that in which resides the illuminating power ; the red 
being said to be the heating ray, and the violet or blue the 
chemical. But this is true only in a certain limited sense, 
especially as regards tlie chemical action of the blue rays. 
While it is the case that certain salts of silver, such as the 
chloride, become darkened by the blue rays, while they do not 
undergo visible change by similar exposure to the red or yel- 
low. Yet is it now known that other salts of the same metal, 
when exposed to the solar spectrum, are more quickly changed 
by the red than by the blue. Examples of this will be found 
adduced in the chapter on Heliochromy, or the production of 
photographs in natural colors. 

It is correct to say that all the rays induce chemical change, 
although in the chemicals employed in the ordinary practice 
of photography, those in the region of the blue and violet end 
of the spectrum are the most active. In the early days of the 
art, before the value of bromine as a photographic agent be- 
came known, yellow and red were almost synonymous with 
black, while articles of a blue color appeared white in the 
photograph. This is now no longer the case. 

All who desire to become thoroughly acquainted with the 
properties of light in connection with photography, should 
possess faculties for projecting the solar spectrum upon the 
table at which they operate. This is readily accomplished by 
admitting a ray of sunlight through a small aperture in an 
opaque window shutter, and interposing in its path a triangular 
glass prism, by which the beam will be decomposed into the 
prismatic colors, and be deflected out of its course, A ther- 
mometer placed in the spectrum thus formed rises most 
rapidly in the red rays ; the yellow gives the highest degree 
of illumination as proved by the sense of sight, while a piece 
of sensitive photographic paper, as commonly employed in 
printing, undergoes no change in the red, scarcely any in the 
yellow, but darkens with great rapidity in the violet. 

EEFKACTION OF LIGHT, AND LENSES, 

A ray of light, in its passage through any transparent 
medium, travels in a straight line as long as the density of the 
medium continues unchanged. But if the density varies, be- 
coming either greater or less, then the ray is refracted, or bent 
out of the course which it originally pursued. The degree to 
which the refraction or bending takes place depends upon the 



328 



OPTICS OF PHOTOGRAPHY. 



nature of tlie new medium, and in particular upon its density as 
compared with that of the medium which the ray had pre- 
viously traversed. Hence water refracts light more power- 
fully than air, and glass more so than water. 

The following diagram illustrates the refraction of a ray of 
light : 




Fig. 16. 



The dotted line is drawn perpendicularly to the surface, and 
it is seen that the ray of light on entering it is bent towards 
this line. On emerging, on the other hand, it is bent to an 
equal extent, away from the perpendicular, so that it proceeds 
in a course parallel to, but not coincident with, its original di- 
rection. If we suppose the new medium, in place of being 
more dense than the old, to be less dense, then the conditions 
are exactly reversed — the ray is bent away from the perpendic- 
ular on entering, and towards it on leaving. 




Fig. 17. 



Fig. 18. 



It must be observed that the laws of refraction apply only 
to rays of light which fall upon the medium of an angle ; if 
they enter perpendicularly — in the direction of the dotted 
lines in the last hgure — they pass straight through without 
suffering refraction. 

Notice also, that it is at the surface of the bodies that the 
deflecting power acts. The ray is bent on entering and bent 



LENSES. 



329 



again on leaving ; but whilst within the medium it continues 
in a straight line. Hence it is evident that by variously modi- 
fying the surfaces of refractive media the rays of light 
may be diverted almost at pleasure. This will be rendered 
clear by a few simple diagrams, "^ 

In the figures following, the dotted lines represent perpen- 
diculars to the surface at the point where the ray falls, and it 
is seen that the usual law of bending towards the perpendicu- 
lar on entering, and away from it on leaving the dense medium, 
is in each case correctly observed. ""^ rrz,' 

±ig. 18, m the last page, termed a prism, bends the ray per- 
manently to one side ; Fig. 19, consisting of two prisms placed 
base to base, causes rays before parallel to meet in a point ; and 
conversely, Fig. 20, having prisms placed edge to edge, diverts 
them further asunder. 





Fig. 20. 

The various Formnn of Lenses. — The phenomena of the re- 
fraction of light are seen in the case of curved surfaces in the 
same manner as with those which are plane. 

Glasses ground of a curvilinear form are termed lenses. The 
figures given on the following page are examples. 

Fig. 21 is a biconvex lens; Fig. 22, a biconcave lens; and 
Fig. 23, a meniscus lens. 

As far as regards their refractive powers, such figures may 
be represented, nearly, by others bounded by straight lines, and 
thus it becomes evident that a biconvex lens tends to condense 
rays of light to a point, and a biconcave to scatter them. A 
mensicus combines both actions, but the rays are eventually bent 
together, the convex curve of a meniscus lens being always 
greater than the convave. 

The Foci of Lenses. — It has been shown that convex lenses 
tend to condense rays of light and bring them together to a 
point. This point is termed " the focus" of the lens. 



330 



OPTICS OF PHOTOGRAPHY. 



The following laws as regards the focus may be laid down : — 
That rays of light which are pursuing a parallel course at the 
time they enter the lens are brought to a focus at a point nearer 
to the lens than diverging rays. The rays proceeding from 
very distant objects are parallel; those from objects near at 
hand diverge. The sun's rays are always parallel, and the di- 
vergence of the others becomes greater as the distance from the 
lens is less. 




Fig. 21. 



Fig. 22. 



Fig. 23. 



The focus of a lens for parallel rays is termed the " principal 
focus," and is not subject to variation ; this is the point re- 
ferred to when the focal length of a lens is spoken of. When 
the rays are not parallel, but diverge from a point, that point 
is associated with the focus, and the two are termed " conjugate 
foci." 




In the last diagram, A is the principal focus, and B and 
are conjugate foci. Any object placed at 13 has its focus at 0, 
and conversely when placed at C it is in focus at B. 

Therefore, although the principal focus of a lens (as deter- 
mined by the degree of its convexity) is always the same, yet 
the focus for objects near at hand varies, being longer as they 
are brought closer to the lens. 

Formation of a Luminous Imnge by a Lens. — As the rays 
of light proceeding from a point are brought to a focus by 



LENSllS. 



331 



means of a lens, so are they wlienthey proceed from an object, 
and in that case an image of the object is the result. 

In orler that the course pursued by pencils of rays proceed- 
ing from an object may be easily traced, the lines from the 
barb of the arrow in Fig. 25 are dotted. Observe that the object 
is necessarily inverted, and also that those rays which traverse 
the central point of the lens, or the centre of the axis, as it is 
termed, are not bent away, but pursue a course either coin- 
cident with, or parallel to, the original, as in the case of refract- 
ing media with parallel surfaces. 

Enlarged and Reduced Images. — When an object is placed 
at some distance in front of a lens, an image is formed smaller 
than the object ; but if the object be advanced nearer to the 
lens the image increases in size until it becomes, iirst equal to, 
and then larger than, the original, the focus at the same time 
receding to a greater distance from the lens. 



/f\^^-:-- 




Fig. 25. 

Thus, if a photographic portrait be approximated to a lens 
an image of the life size may be thrown upon a screen, just as 
the rays proceeding from the living model were in the first in- 
stance employed to form the portrait. The two planes, in 
fact, of the object, and of the image, are strictly conjugate foci 
(page 396), and it is immaterial from which of the two, anterior 
or posterior, the rays of light proceed. 

Amplifying instruments, like Woodward's patent solar 
camera are constructed upon the above principle, and are 
found useful in enlarging photographs to three or four times 
their original diameter. A loss of definition always results in 
this process, which may conveniently be remedied by the pencil 
of the artist. 

Lenses of Long and Short Focus. — A lens of short focus 
placed at a given distance from an object forms a small image, 



332 



OPTICS OF PHOTOGKAPHY. 



the rays being strongly refracted. If the lens be brought 
nearer to the object, the image becomes larger, as above 
shown ; but at a certain point the approximation must be 
stopped, or the lens will be strained, as the opticians term it, 
and distortion will result. Therefore lenses are purposely con- 
structed of a longer focus when a large image is required. 

Long-focus lenses for taking large pictures have usually a 
considerable diameter, but it will be understood that the size 
of a lens has nothing whatever to do with the size of the 
image. The focal length of the lens, at a given distance, de- 
termines the size of the image ; nevertheless, since the lumin- 
ous rays are diffused over a large space when the focus is long, 
the optician is compelled in such a case to increase the aper- 
ture of the glass in order to transmit more light. 

The focal length of a single actinic combination is usually 
found by focusing a distant object, and then measuring the 
distance from the back of the lens to the refracted image on 
the screen. This distance represents the focal length of the 




Fig. 26. 



lens. It is not strictly accurate, but near enough for all prac- 
tical purposes. The focal length of a double or triple combi- 
nation of lenses cannot be thus estimated, because the point 
from which it is measured is situated somewhere in front of 
the back combination ; hence a more complicated process of 
measurement must be adopted to determine what is called in 
this case the "equivalent focus," or the distance from the 
screen at which a single lens must be placed in order to give 
the same sized image. This will be treated of farther on. 

The photographic camera, in its essential nature, is an ex- 
tremely simple instrument. It consists merely of a dark 
chamber, having an aperture in front in which a lens is in- 
serted. The accompanying figure (26) shows the simplest form 
of camera. 

The body is represented as consisting of two sliding por- 
tions ; but the same object of lengthening or shortening the 
focal distance may be attained by making the lens itself mov- 



LENStes. 333 

able. A luminous image of any object placed in front of the 
camera is formed by means of the lens, and received upon a 
surface of ground glass at the back part of the instrument. 
When the camera is required for use, the object is focused 
upon the ground glass, which is then removed, and a slide con- 
taining the sensitive layer inserted in its place. 

The luminoas image, as formed upon the ground glass, is 
termed the " field " of the camera ; it is spoken of as being 
flat or curved, sharp or indistinct, etc. These and other pecu- 
liarities depending upon the construction of the lens will now 
be explained. 

Chromatic Aberration : Visual and Aotinio Focus. — The 
outside of a biconvex lens resembles the sharp edge of a prism, 
and necessarily produces decomposition in the white light 
which passes through it. The luminous image of an uncor- 
rected lens is in consequence bounded by colored figures. 

The action of a prism in separating white light into its con- 
stituent rays may be simply explained. The indigo and violet 




Fig. 27. 

rays are more bent away than the yellow and red, and conse- 
quently tliey are separated from them, and occupy a higher 
position in the spectrum. (See Fig. 27.) A little reflection 
will show that in consequence of this unequal refrangibility of 
the colored rays, white light must invariably be decomposed on 
entering any dense medium obliquely. This is indeed the 
case ; but if the surfaces of the medium are parallel to each 
other the effect is not seen, because the rays recombine on their 
emergence, being bent to the same extent in the opposite direc- 
tion. Hence light is transmitted colorless through an ordinary 
pane of glass, but yields the tints of the spectrum in its pas- 
sage through a prism or a lens, where the two surfaces are in- 
clined to each other at an acute angle. 

The same causes which produce chromatic aberration in a 
lens, tend also to separate the chemical from the visual focus. 
The violet and indigo rays are more strongly bent than in the 
yellow, and still more than the red ; consequently the focus for 
each of those colors is at a different point. The preceding 



334: OPTICS OF PHOTOGEAPHY. 

diagram shows this. Y represents the focus of the violet ray, 
Y of the yellow, and R of the red. Hence, as the chemical 
action corresponds more to the violet, the most marked actinic 
effect would be produced nearly at Y. The luminous portion 
of the spectrum, however, is the yellow, consequently the 
visual focus is at Y. 

Photographers have long recognized this point ; and there- 
fore, with ordinary lenses, not corrected for actinism, rules are 
laid down as to the exact distance which the sensitive plate 
should be shifted away from the visual focus in order to ob- 
tain the greatest amount of distinctness of outline in the image 
impressed by chemical action. 

Actinic aberration is corrected by combining two lenses cut 
from varieties of glass which differ in their power of separat- 
ing the colored rays. These are generally the dense flint glass 
containing oxide of lead, and the light crown glass. Of the 
two lenses, the one is biconvex and the other biconcave ; so 



— > 




Fig. 28. 

that when fitted together they produce a compound actinic 
lens of a meniscus form, as shown in Fig. 28. 

The first lens in this figure is the flint, and the second the 
crown glass. Of the two the biconvex is the most powerful, 
so as to overcome the other, and produce a total of refraction 
to the required extent. Each of the lenses is made from glass 
of different dispersive power to that of its coadjutor ; and the 
effect of passing the rays through both is, by overlapping the 
colored spaces, to unite the complementary tints, and to re- 
form white light. 

Spherical or Axial Aierratio7i. — Spherical aberration is 
the property possessed by lenses which are segments of spheres 
of refracting rays of light unequally at different parts of their 
surfaces. Fig 29 shows this in an exaggerated degree. 

Observe that the dotted lines which fall upon the circiirafer- 
ence of the lens are brought to a focus at a point nearer to the 
lens than those passing through the centre ; in other words, 
the outside of the lens refracts light more powerfully. This 
causes a degree of confusion and indistinctness in the image, 



LENSES. 335 

from various rajs crossing and interfering with each other. 
In correcting for spherical aberration, the lirst point to be 
attended to is the curve of the lens itself. A biconvex lens, 
as shown in the (Fig. 28) diagram, is the worst form. The plano- 
convex and the meniscus lenses are preferable, and for land- 
scapes the latter is very commonly employed. 

But spherical aberration, even in a lens which is badly made 
as regards form, may be remedied in another way, viz., by 
using a diaphragm or stop, so as to cut off a portion of the 
light, and prevent the same pencil of rays from falling both 
on the centre and on the edge of the lens. As the action of a 
stop is more fully explained a few pages in advance, we defer 
the further consideration of it for the present. 

Curvature of the Field. — In using a camera mounted with 
a meniscus lens, it will be observed that when the centre of the 
field has been focused, the outer portions of the image appear 
indistinct, whilst if the ground glass be pushed in a little, the 




Fig. 29. 

outside becomes sharp, but the centre is thrown out of focus. 
This defect is sometimes attributed to spherical aberration, but 
incorrectly so, because indistinctness from that cause is seen 
alike at every portion of the field. The want of definition 
which can be remedied by shifting the position of the f ocusing- 
screen we refer rather to curvature of the field, or to the fact 
that the image of a flat surface, formed by a lens, produces not a 
plane, but a portion of a hollow sphere, as shown by the in- 
verted arrow, page 832. 

In the diagram (Fig. 30) the centre line running at right angles 
to the general direction of the lens is the axis; an imaginary 
line, on which the lens might be rotated as a wheel is turned 
on its axis. The lines A A represent rays of light falling paral- 
lel to the axis ; and the dotted lines, others which have an 
oblique direction ; B and C show the points at which the two 
foci are formed. Observe that these points, although equi- 
distant from the centre of the lens, do not fall in the same ver- 
tical plane, and therefore they cannot both be received simul- 



336 



OPTICS OF PHOTOGRAPHY. 



taneously upon the ground glass of the camera, which would 
occupy the position of the perpendicular double line in the 
diagram. 

The defects due to curvature of the field might be avoided 
hy bending inwards the glass or paper used to sustain the sensi- 
tive iodide, in such a way as to meet the image, as is done in 
the apparatus for Sutton's panoramic lens ; but practically this 
plan, though possessing some advantages, is in other respects 
objectionable. The image itself therefore mast be flattened 





Fig. 30. 

out, and this can be effected by means of the diaphragms be 
fore alluded to in the paragraph on spherical aberration ; the 
position and use of which will be shown in the following pages. 
Variation of Focus for J^ear and Distant Objects. — 
Lenses employed with the full aperture, do not render near and 
remote objects sharp upon the ground glass of the camera at 
the same time. If the foreground is in focus, the lens must be 
thrown inwards to make the distance clear, and vice versa. 




Fig. 31. 

This is a necessary consequence of the focal plane of any ob- 
ject varying with the position of the object in regard to the 
lens. 

The action of a diaphragm or stop, already spoken of under 
the head of " spherical aberration" and " curvature of the 
field," is seen to still greater advantage in remedying the de- 
fect now complained of. 

Fig. 3 1 represents rays of light radiating in all directions 
from a luminous point, and falling on the entire surface of a 



LENSES. 337 

lens ; Fig. 32, the action of the diaphragm in intercepting that 
portion of the rays which otherwise would impinge upon the 
outside of the glass. The focus in each case is at the point 
where the rays cross each other after refraction. ]^ow in Fig. 
31 suppose the grayed screen, represented by the dotted line, 
to be advanced nearer to the lens, or to be withdrawn from it, 
even in the least degree, the image would immediately be out 
of focus ; out in the second figure, the rays run so nearly paral- 
lel, that the effect of a slight change of position would not be 
perceptible. The refracted pencil is, as it were, sharpened and 
drawn out, so that the focus has considerable depth, and is no 
longer confined to a single plane. The first figure represents 
what is termed a large angular pencil of rays, and Fig. 32 a 
small angular pencil ; the latter alone can be employed when 
near and distant objects are to be simultaneously depicted. 

The focal variation for near and distant objects is much 
more considerable when lenses of very long focus are employed, 
and (always supposing the distance from the object to remain 
the same in the two cases) it becomes less evident with lenses 
of short focus. Hence the small lenses used for stereoscopic 
photography have usually some depth of focus with a compara- 
tively large aperture. 

The optics of photography differs from that of the telescope 
or microscope. It comprises the formation of an image of 
such objects as have an extended range from side to side, or 
above and below that point to which the lens is directed, such 
image to be formed upon a flat plate, and made to cover an 
area of considerable extension. Telescopic optics, on the 
other hand, has relation to the formation of a sharp image in 
the axis of the lens only of any object situated in or near its 
axis, the sharpness being such as to permit of the imaace being 
examined through a powerful magnifier. While telescopic 
optics thus has reference to the axial transmission of light 
through a lens, in photographic optics, on the contrary, such 
conditions must be sought for as will secure the sharp distribu- 
tion of a scene or figure over a certain area designated the field 
of delineation. To secure this the light must be made to 
pass through the lens in an oblique as well as axial manner. 

Photographic lenses may be divided into four classes — the 
single achromatic landscape lens, the portrait lens, the group 
or copying lens, and the wide-angle non-distorting or architect- 
ural lens. Between each of these classifications it is difficult 
to draw a hard-and-fast line, seeing that some objectives are 
now constructed which fulfill more than one of the requirements 
named. 



338 



OPTICS OF PHOTt)GRAPHY. 



The Single Landscape Lens. — For ordinary landscapes no 
lens or form of objective has yet been introduced which sur- 
passes the single achromatic lens, which, when properly con- 
structed, gives pictures possessing sharpness and brilliance, in- 
cludes a wide angle of subject, and works with a reasonable 
degree of rapidity. The form of the single achromatic land- 
scape lens is shown in Fig, 33, in which an achromatic 



/f*^.. 



\ 



Fig. 33. 

meniscus is placed at one end of the tube, there being a 
diaphragm in front of it. 

At this stage we may fittingly introduce a diagram which 
explains the useful part subserved by the diaphragm or stop 
when placed in front of a landscape lens. 

In this diagram a number of rays O, Z, and those between 




Fig. 34. 

them, are represented as impinging obliquely upon the flattest 
surface of a lens, L, all of them, however, taking different 
courses as regards their crossing each other after being trans- 
mitted. The image of any object would be quite devoid of 
sharpness, and be hazy, owing to the ray Z being refracted in a 
greater degree than any others of those between it and the 



LENSES. 



339 



upper boundary ray O. If tlie transmitted ray Z, instead of 
taking the coarse Z', went straight to 0',the intermediate rays 
doing the same, then would we have a perfect single leris ; 
but such is not the case, owing to the aberration caused by 
the spherical surface of the glass. Spherical aberration, it 
may here be once more remarked, is that property in a lens 
whereby parallel rays falling either obliquely or axially upon 
its surface undergo such different degrees of refraction as pre- 
vent them meeting at one point. 

- The function of a diaphragm, as applied to a single achro- 
matic lens, having its flattest side next to the subject to be de- 
picted, is to prevent any rays from having access to the lens, but 
those which shall be transmitted, both axially and obliquely, 
to a pictorially sharp focus. The manner in which the dia- 
phragm affects this is shown in Fig. 35, in which the axial 




Fig, 35. 

rays are transmitted to a sharp focus, the oblique rays, all but 
those which pass through the diaphragm, and come to a focus 
as shown, being debarred access to the lens by the mounting 
and the diaphragm. 

We shall hereafter speak of other functions of the dia- 
phragm. The subject is here introduced to show in what man- 
ner it becomes necessary to the correct working of a single- 
view lens, whether achromatized or not. 

Lenses are achromatized by a judicious combination of a con- 
cave lens formed of flint glass, with a crown glass convex, the re- 
spected degrees of curvature being so determined as to bring 
all the rays of the spectrum, at any rate the visual and more 
energetic of the chemical ones, to a focus. Were this not done, 
the plane at which by aid of the ground glass of the camera, 



340 



OPTICS OF PHOTOGRAPHY. 



the image was seen to be sharp would be somewhat distant 
from that plane at which the more violet rays were brought 
to their focus, and as the picture is produced by these latter 
rays it would necessarily be out of focus. By securing the 
coincidence of the chemical and visual focus, the picture which 
is depicted upon the ground glass or focusing screen of the 
camera is that which is secured upon the chemically prepared 
surface of the sensitive plate. 

The forms which the concave flint and the convex crown 
glass lenses are made to assume depend largely upon the ideas 
of respective makers, some preferring one form and some 
another. The systems in use by one or other of the leading 
lens-makers at the present time are those depicted in Fig. 
36. 




Fig. 36. 

^ |In external form these are all alike, or nearly so ; but when 
their interior construction is observed it is seen that all simi- 
larity ceases. Of these various methods of achromatizing a 
lens, the third and fourth are preferred on account of their 
transmitting to a focus a larger pencil light than the others. 
The first, a bi-concave flint and a bi-convex crown, is the old- 
est form of achromatic lens, the second being merely an ex- 
tension of the same principle of construction, to render the 
lens more of a meniscus form. The third shows an entirely 
diiferent principle in the arrangement of the elementary parts, 
and is that now adopted in almost all of the "rapid" series of 
lenses which will be described in turn. In this, when em- 
ployed as a single landscape objective, the light falls upon the 
crown instead of the flint glass, and the inner or contact sur- 



LENSES. 



341 



faces are more nearly normal to the transmitted pencil than in 
the former forms of lens. The fourth is in a certain sense sim- 
ilar in principle to the third, but possesses this advantage, that 
the flint glass which is nsually soft and liable to become 
scratched or otherwise damaged is enclosed between two 
crowns, and is the most perfect form of single achromatic 
landscape lens that has been constructed, and is shown fully- 
mounted in Fig. 37, which represents Dallmeyer's landscape 
lens. When a single achromatic lens of the kind described — 
whatever its interior form of curvature — is required to include 
a very wide angle of view, or, in other words, to cover a large 
field in comparison with its focus, it is necessary that its exte- 
rior form be that of a deep meniscus and that the diaphragm 
be placed somewhat near to the first surface. This permits of 




Fig. 37. 

the transmission of extremely oblique rays. When the men- 
iscus form is very pronounced, a smaller diaphragm is required 
to correct the spherical aberration than with an objective more 
nearly approaching a plano-convex, in which latter case a 
larger aperture may be employed, thus securing greater rapid- 
ity of action but on a less extended field. The best single 
landscape lenses are those which will, with a large aperture in 
the diaphragm, give a sharp picture over a moderately large 
field ; and with an aperture much reduced, extend that sharp- 
ness over an angle of eighty or ninety degrees on the base 
line. 

It is now requisite we should speak of a defect inherent in 
the landscape lens which, although not affecting its use in the 



342 



OPTICS OF PHOTOGRAPHY. 



production of landscapes, militates against its utility in taking 
copies of anything requiring absolute accuracy, such as maps 
and pictures, or in the delineation of architectural subjects. 
This defect is its inability to produce rectilinearity. As will 
have been ascertained from an inspection of Fig. 35, the mar- 
gin of a lens bends a ray to a somewhat greater extent than 
.any other part of the area of such lens; and as, owing to the 
position of the diaphragm, the rays which come from the sides 
of the subject are transmitted by the margin of the lens, a cer- 
tain amount of bending towards the centre results, it follows 
that a square building, a map, plan, or anything else that is to 
be reproduced, will not be represented precisely as it ought to 
be, but will experience a slight degree of curvature of the 
marginal lines. It is only in or towards the margin of the 
picture that such effect is noticed, but even though not readily 
observable to the eye, especially if the photographer has done 
his part well and with understanding, and has taken care not 






No. 1. 





No. 3. 



to force the lens to include a wide angle of the subject being 
copied, the application of a straight-edged rule will reveal the 
distortion of curvature, by which a square original becomes 
slightly barrel-shaped in the photograph, as shown in No. 2, 
Fig. 38. The distortion here referred to is of quite a different 
nature from another kind, in which lines in a building which 
ought to be perpendicular in the picture are delineated with 
converging lines. This arises from another cause, and is alto- 
gether irrespective of the nature of the lens employed. 

If the lens, together with its mount, were entirely reversed, 
so that its convex surface should be pointed towards the view, 
the diaphragm being next the ground glass, the distortion 
would not be cured, but would assume a precisely opposite 
character to what it did in the former instance. The distortion 
in that case was of a barrel shape ; in this it is of the pincush- 
ion form, as shown in JS^o. 3. 

The entire elimination of all distortion is effected by having 
a lens on each side of the diaphragm, so that the barrel and 



LENSES. 343 

pincushion curves neutralize each other, and meet together in 
one absolutely straiij;ht line, as in ISTo. 1. 

The optical condition for securing freedom from distortion 
consists in this, that every ray which falls upon an objective 
must emerge after transmission in a direction parallel to that 
at which it entered. Symmetry, whether mechanical or opti- 
cal, in a combination, ensures this result, and we shall next 
come to speak of non-distorting combinations. 

It will readily be perceived that under no circumstances 
could the condition described as that requisite for rectilinear 
projection be obtained by the employment of any species of 
single lens, even if it were achromatized. Numerous forms of 
compound lenses have been devised to meet the requirements, 
and with one exception all of them have proved successful so 
far as concerns the giving of straight lines ; that exception is 
the now little-used orthoscopic or orthographic combination, for 
which many claims were at one time made in support of the 




Fig. 39. 

sentiment embodied in the name. But notwithstanding such 
claims, the fact that it did produce distortion could not long be 
gainsaid ; but the curvature of those marginal lines that ought 
to have been straight, was of a nature quite opposite to that 
produced by the single achromatic, an expansion rather than a 
contraction of the proportions in the drawing taking place, in 
proportion as it was distant from the centre. The combination 
(Fig. 39) consists of a somewhat flat meniscus, achromatic at 
the outer end of a short mount, the convex side out ; a smaller 
and negative achromatic being placed a little distance behind. 
This negative lens, which consists of a biconcave crown and a 
meniscus flint, corrects the aberration of the outside one, 
lengthens the focus and flattens the field. 

While failing for the specific purpose for which it was 
originally constructed, the orthoscopic lens, from its capability 
of working with a large aperture, answers admirably for out- 



344 



OPTICS or PHOTOGRAPHY. 



door groups, or even portraits in a good liglat, and with very 
sensitive plates. It is also useful for producing an image of 
larger size than could be obtained by any other lens on a camera 
of a given length. The size of an image of any object in the 
negative depends upon the focus of the lens by which it has 
been taken. This focus is measured from, and determined by, 
the position of its optical centre ; the optical centre of a well- 
constructed single achromatic landscape lens is rather nearer to 
the ground glass of the camera than the lens itself, but in the 
orthoscopic combination it is outside of the lens entirely, so 
that with a given length of camera, a much larger image can, 
as we have stated, be obtained by this lens than by any other. 
This is a property of much value. 

The " triple achromatic" (Fig. 40) of Mr. J. H. Dallmeyer, 
was a decided improvement upon the orthoscopic lens, inas- 




Fig 40. 

much as by it pictures quite free from distortion were obtained. 
This lens served a useful purpose in its day, but like the other 
it is now falling into disuse, having been supplanted by more 
effective and simpler combinations. It consists of a slightly 
meniscus achromatic, convex side out ; a little distance behind 
this is a nearly plane meniscus concave achromatic, a larger 
lens of the same form as that in the front being placed at the 
back end. 

Apropos of the triple achromatic, Mr. Grubb has ap- 
plied to single landscape lenses an additional front, com- 
posed of simple lenses of a form nearly similar to the two 
aclu-omatics in Mr. Dallmeyer's combination, viz., piano- 



LENSES. 



345 



convex and plano-concave, the size of which, in connec- 
tion with any achromatic landscape, cures its distortion 
and renders it suitable for architectural purposes. Both 
of these lenses are formed of crown glass, and possess such 
curves as to make the two, when combined, in effect like a 
piece of plain glass, in the sense of their having no magnifying 
power. They are placed immediately in front of the diaphragm 
of the lens, the rounded surface of the plano-convex lens to the 
outside. This causes such a degree of displacement of the 
marginal rays as to cure distortion. 

While on the subject of neutralizing the distortion of lenses 
which produce this bad quality, it may be said that the pin- 
cushion distortion of the orthoscopic lens may be cured by a 
simple expedient devised by the editor of this work. It con- 
sists in interposing a thick sheet of plate glass immediately in 
front of the sensitive plate, and as close to it as possible. The 




Fig. 41. 

rationale of its action is this : a ray falling on its surface at a 
right angle is transmitted without being deflected, but in pro- 
portion to the obliquity of the incident ray so is it refracted in 
course of transmission, a fact quite apparent upon laying a 
thick slab of glass down upon printed matter and looking 
obliquely through it. The inward deflection caused by this 
oblique incidence balances the outward departure from rectili' 
nearity caused by the form of the lens, the photographic result 
being freedom from distortion. 

The death knell to the construction of several combination 
of lenses was struck when, almost simultaneously Steinheil, of 
Munich, and Dallmeyer, of London, introduced each a lens, 
which, although possessing slight differences in construction, 
are in effect and general configuration so nearly allied as to be 
practically alike. 



346 OPTICS OF PHOTOGKAPHY. 

The aplanat of Steinheil is constructed of two kinds of flint 
glass, a light and a dense kind, by which achromatism is effected 
in the same manner as by means of flint and crown. The 
rapid rectilinear of Dallmeyer is composed of flint and crown. 
The diagram, Fig. 41, explains the formation of both in a 
manner sufficiently plain to be understood. They work sharply 
with an aperture equalling about a seventh or eighth of the 
focus, and are thus well adapted for groups and portraits in a 
good light and with sensitive plates ; while being symmetrical 
both in an optical and mechanical sense they give pictures free 
from distortion. 

This class of lens, which is now very extensively manu- 
factured, wdth certain slight modifications, is to be met with 
all over the world under various names, to produce which both 
the dead and the living languages have been placed under heavy 
contribution. When used with a small diaphragm it includes 
an angle of considerable extent, although, owing to the com- 
parative flatness of the exterior surfaces, and the length of the 
tube in which the lenses are mounted, they can scarcely rank 
as wide-angle objectives. But even to this requirement they 
lend themselves with great degree of pliancy, for the editor 
has not unfrequently taken views which include an angle of 
ninety degrees by mounting a pair of these lenses tolerably 
close together, and inserting a diaphragm between them. 
Their internal curvature affords facilities for the transmission 
of a very oblique ray. 

All the lenses constructed on the " rapid " principle, and ac- 
cording to the system shown in the last diagram, must be 
formed of a peculiar kind of glass, possessing a greater degree 
of density than that of ordinary flint and crown. Bnt an 
American optician, Mr. R. Morrison, has by means of an en- 
tirely different method of achromatizing, secured an angularity 
ot aperture, and therefore a rapidity, equalling that of the 
rapid lenses mentioned by means of the flint and crown glass 
so long employed in other optical productions. The nature of 
this objective will be underst )od from the following descrip- 
tion of one of ten inches in back focus, which was carefully 
examined. The exterior lens of either combination consists of 
a plano-convex crown, the margin of which is in contact with 
a double concave flint. The radius of the former is four 
inches, that of each surface of the latter being 20.00 and 10.20 
respectively, the flatter of the two surfaces being next to the 
plane surface of the crown-glass lens. The objective is 
symmetrical, both front and back combinations being alike. 



LENSES. 347 

In other objectives by the same maker and of the same type, 
but more particularly in some exceeding twenty inches in 
focus, while both crown glasses are plano-convex, both com- 
binations are dissimilar as regards curves. Owing to the flat- 
ness of the inner curves, this system favors to a greater ex- 
tent than others, the formation of large lenses at a moderate 
cost. 

WIDE-ANGLE NON-DISTOETING LENSES. 

In wide-angle lenses of the kind now to be considered it is 
essential that the meniscus form be very deep, so as to present 
a surface nearly at a right angle to the incident and emerging 
ray, whether it be an axial or oblique one. To meet this re- 
quirement the American globe lens was devised. In it the 
globular idea was carried out (see Fig. 42) in its entirety, as the 
radius of the outer surface A B was measured from the centre 
E of the combination. It gave pictures free from distortion, 




Fig. 42. 

and included a tolerably wide angle, but often faulty from a 
flare spot in the centre, the result of a too close adherence to the 
globe form, for had the lenses been placed closer to C D, not 
only would a wider angle of view have been included, but the 
conditions under which the flare spot referred to was produced 
would have been disturbed. 

It is worthy of notice that Morrison's wide-angle lens, which 
is now extensively used in America, and gives pictures of a 
high degree of optical excellence, is erected upon what we may 
term the ruins of the globe lens. Its front consists of a deep 
meniscus achromatized in the manner of the globe, its back 
lens consisting of a simple non achromatic crown meniscus, of a 
focus a little longer than its confrere. The diaphragm is 
placed between them in the ratio of their foci. Notwithstanding 
the non-achromatizing of the back lens, and the absence of any 



348 



OPTICS OF PHOTOGEAPHT. 



trace of over correction of the front, this combination has its 
visual and chemical focus practically coincident. 

Both Dallmeyer and Steinheil have constructed wide-angle 
lenses on the same principles as those which distinguish their 
rapid lenses already spoken of. But in the one case the rapid 
was an outcome, in a sense, of the wide-angle rectilinear ; while 
in the other, the rapid aplanat was the precursor of the wide 
angle. The former is illustrated in the diagram (Fig. 43). 

The wide-angle aplanat is small in diameter, and of a deeply 
curved form. Compared with their diameter, the lenses are 
very thick, and are mounted so closely together as not to al- 
low much more space between them than suffices for the in- 
sertion of a diaphragm. Like its progenitor, it is formed of 
two kinds of flint-glass. 

Those wide-angle lenses, just described, are typical of most 
others now being manufactured. 




Fig. 43. 



In the mounts of wide-angle landscape lenses a great im- 
provement has -been effected by Messrs. Eoss & Co., who have 
constructed quite a large series of symmetrical lenses of var- 
ious foci, but all of which have the mount of the same dia- 
meter, and are thus capable of screwing into the same flange. 
This is a great convenience to the tourist or landscape artist. 

POKTKAIT LENSES. 

"We now come to speak of that class of lenses which are 
more used than any other, that which, on account of its special 
qualities, is termed the portrait lens. The exceptional prop- 
erties of this lens consist in its possessing a very large aperture 



LENSES. 349 

or area of surface in comparison with its focus, this property 
being designated angular aperture. 

Its aberrations are also corrected in a manner so perfect as to 
define with extreme sharpness, without a diaphragm ; the 
practical result arising from such properties is that a sharp 
portrait can be taken in a very brief period of time and in a 
subdued light. 

A lens possessing a large angular aperture is directly anti- 
thetical to a lens which includes a large angle of view. The 
latter embraces a wide extent of subject, but must be used 
with a small aperture; whereas the former embraces only a 
limited amount of subject, but works with a large aperture. 
Two lenses may be precisely similar, so far as regards diameter, 
but if one be of shorter focus than the other, its angular aper- 
ture will be greater. This property confers the great advan- 
tage of rapidity of action. A lens only one inch in diameter 
may work more rapidly than one of four inches, because of 
its focus in relation to its aperture being shorter, and admit- 
ting a ray of greater intensity. 

By the skill of the opticians, lenses are now constructed 
having an aperture so great as to equal twice their foci. Such 
lenses are only of limited utility when worked without a dia- 
phragm, for they cover only a small portion of the plate 
sharply, while their depth of defining power has only little 
range ; but owing to the flood of light transmitted, they work 
with an extreme degree of rapidity, and are invaluable in the 
taking of portraits of children or paralytic persons. But both 
the field of delineation and the depth of definition may be ex- 
tended by the insertion of a diaphragm, these advantages being 
obtained at the expense of illumination, and consequently of 
increased exposure. 

In the construction of a portrait objective, it is necessary 
that the front lens be an achromatic of plano-convex form, or 
nearly so, and that its convex side be next to the object to be 
taken. Used alone in this way it would produce a tolerably 
sharp and bright image without any diaphragm, but such 
sharpness would be confined to a limited area in the centre of 
the picture. A second, or back lens, is therefore made to 
work in conjunction with the front one for the twofold pur- 

Eose of securing a distribution of sharp definition over a larger 
eld, and incidentally by shortening the focus, adding ma- 
terially to rapidity of action. 

The aberrations of the front lens, especially for oblique rays, 
are such as to render it altogether impossible that these can be 



350 



OPTICS OF PHOTOGKAPHT. 



transmitted to a focus, especially on the same plane as that to 
which the axial rays converge, hence the back lens must con- 
sist of a combination of two simple lenses, the materials and 
curvatures of which are so adjusted as not only to form an 
achromatic in itself, but to possess so great an amount of 
negative spherical aberration as shall neutralize the positive 
aberration of the front lens, and exercise so much control 
over the oblique pencils as not only to bring them all to a 
focus, but also to lengthen them out to such an extent as to 
cause that focus to fall upon the same plane as the central rays, 
and thus yield that great boon to the portraitist, a sharp flat 
field. 

A back lens, or, more correctly, a pair of lenses, of a port- 
rait combination, cannot when used alone give a sharp image, 




Fig. 44. 

owing to its negative aberration, by which its margin is of a 
longer focus than its centre. 

The discovery of this balancing of the aberrations, and con- 
sequent production of a portrait combination, is due to Pro- 
fessor Petzval, the eminent mathematician of Vienna, and the 
figure (44) shows the portrait lens in the form in which it was 
invented by him, and is manufactured at the present day. 

This portrait objective is only suitable under certain cir- 
cumstances for views or out-of-door pictures ; these are, its 
being worked with full aperture, and the shielding from it of 
all extraneous light, especially that from a bright sky. By 
making use of a small diaphragm, although all the depth and 
sharpness over a larger field arising from a diaphragm is se- 
cured, yet will there be a troublesome fiare or light spot in the 



LENSES. 



35 L 



centre of the picture, that will spoil the appearance of the 
picture. This is not observed when the lens is worked with 
its open aperture. When used for copying pictures, or other 
work inside of the studio, the flare spot is not produced, no 
matter how small may be the aperture in the diaphragm. 

There is another way, different from that described, by 
which the posterior combination of a portrait objective may be 
constructed. It was first applied by Mr. Dallmeyer, and in it 
both the form and relative positions of the elementary glasses 
are changed. A meniscus crown is placed close to, although 
not in contact with, a concavo-convex flint lens, the exterior 
form of the two being slightly meniscus. The cut shows the 
nature of this combination. 




Fig. 45. 

The two inner or contiguous surfaces of the back pair can- 
not be placed in optical contact by means of transparent ce- 
ment in the same way as the front ones, owing to a dissimi- 
larity of curvatures, by which is obtained the requisite amount 
of negative spherical aberration to flatten the field. When 
the back elements are placed as close together as possible, the 
objective defines sharply ; but by unscrewing them so as to 
effect ever so slight a degree of separation, then does the sharp 
and crisp definition give way to definition of a lower order 
which is useful for many purposes. 



CHAPTER XXIII. 

ON BINOCULAR VISION AND THE STEREOSCOPE. 

An object is said to be "stereoscopic" {(TtspsoS, solid, and 
ffkOTrsGo, I see) when it stands out in relief, and gives to the 
eye the impression of solidity. 

This subject was first explained by Professor "Wheatstone 
in a memoir on binocular vision, published in the " Philosoph- 
ical Transactions" for 1838; in which he shows that solid 
bodies project different perspective figures upon each retina, 
and that the illusion, of solidity may be artificially produced 
by means of the " stereoscope." 

The phenomena of binocular vision may be simply stated as 
follows : — If a cube, or a small box of an oblong form, be 
placed at a short distance in front of the observer, and viewed 
attentively with the right and left eye separately and in suc- 
cession, it will Ije found that the figure perceived in the two 
cases is different ; that each eye sees more of one side of the 
box, and less of the other ; and that in neither instance is the 
effect exactly the same as that given by the two employed con- 
jointly. 

A silver pencil case, or a pen-holder, may be used to illus- 
trate the same fact. It should be held at about six or eight 
inches distant from the root of the nose, and quite at right 
angles to the face, so that the length of the pencil is concealed 
by the point. Then, whilst it remains fixed in this position, 
the left and right eye are to be alternately closed ; in each case 
a portion of the opposite side of the pencil will be rendered 
visible. 

The diagrams on the next page exhibit the appearance of a 
bust as seen by each eye successively. 

Observe that Fig. 47, which represents the impression re- 
ceived by the right eye, is more of a full face than Fig. 46, 
which, being viewed from a point removed a little to the left, 
partakes of the character of a profile. 

The human eyes are placed about 2|- inches, or from that to 
2|- inches, asunder ; hence it follows that, the points of sight 
being separated, a dissimilar image of a solid object is formed 



THE STEREOSCOPE. 353 

on the retina of each eye. We did not however see two images, 
but a single one, which is stereoscopic. 

In looking at a picture painted on a flat surface the case is 
different ; each of the eyes, as before, receives an image, but 
these images are in every respect similar; consequently the 
impression of solidity is wanting. A single picture, therefore, 
cannot be made to appear stereoscopic. To convey the illu- 
sion, two pictures must be employed, the one being a right and 
the other a left perspective projection of the object. The 
pictures must also be so arranged, that each is presented to its 
own eye, and that the two appear to proceed from the same 
spot. 




Fig. 46. Fig. 47. 

I^The reflecting stereoscope, employed to effect this forms 
luminous images of the binocular pictures, and throws these 
images together, so that, on looking into the instrument, only 
a single image is seen, in a central position. It should, how- 
ever, be understood, that no optical arrangement of any kind 
is indispensably required, since it is quite possible, with a little 
effort, to combine the two images by the unaided organs of 
vision. The following diagram (Fig. 48) will make this 
obvious : 



Fig. 48. 

The circles A and B represent two wafers which are stuck 
on paper at a distance of about 1|- inch from each other. They 
are then viewed, either by squinting strongly, until the right 
eye looks at the left wafer, and the left eye at the right wafer, 



354 BINOCULAR PHOTOGRAPHY. 

or else by focusing the eyes for a distance beyond the wafers, 
which is easily effected by looking at a point midway between, 
but giving attention to the indistinct wafers on each side, until 
the right eye at length looks at the right, and the left eye at 
the left wafer ; in both cases the two wafers at first appear 
double, four images being seen, of which two gradually ap- 
proach until they coalesce, and the resulting image appears in 
the first case in front, and in the latter case behind the 
paper. The mode in which coalescence is effected in both 
cases may be thus explained. 

The perception of a single object, while using two eyes in 
ordinary vision, is due to the eyes being directed to the object 
in such a manner that the picture formed in each eye falls on 
similar parts of the retina. If, while viewing an object, one 
of the eyes be pressed by the finger, the object becomes 




Fig. 49. Fig. 50. 

R and L represent the two eyes, and A and B the two wafers. 

double, from the position of the two retinas not correspond- 
ing. From the same cause, on first trying to fix the two eyes 
on the two wafers, they appear double until the optic axis of 
each eye (or a line of indefinite length drawn through the 
centre of the pupil from the central and most distinctly see- 
ing part of the retina) is directed fully to each wafer. When 
this is the case a similar picture falls on similar parts of 
each, and the two objects are perceived by the mind as one. 
The other less distinct images, one on each side of the prin- 
cipal one, are due to each eye perceiving its neighbor's 
wafer, but on points of the eyes that do not correspond. 

The cause of the combined image standing out in front or 
behind the paper is due. to the mind always referring the 
place of an object to the point where the optic axes meet, 



THE STEKEOSCOPE. 



355 



and when the two wafers are united by squinting, this point is 
in front, and when by distant focusing, behind the paper. 
This will be evident from the diagrams on preceding page. 




Fig. 51. 

Fig, 49 represents union by squinting ; the optic axis of 
each eye is directed to the opposite wafer, and the image is 
referred by the mind to P in front, where the axes cross. Fig. 
60 shows the ordinary way of viewing with the stereoscope, 




Fig. 53. 

where each eye is directed to its own wafer. Here the optic 
axes meet beyond the wafers at P, and the compound image 
is perceived in the same place. 



356 BINOCULAR PHOTOGRAPHY. 

If, instead of two similar objects as wafers, the right and 
left perspective views of an object, such as the follow- 
ing of an hollow hexagonal cone, are viewed in a simi- 
lar manner a sensation of depth and elevation is obtained as 
soon as the images coalesce, the different parts of the cone 
appear at different distances from the observer, and what is 
more singular the appearances are reversed bj the two modes 
of causing the pictures to coalesce. 

The diagram. Fig. 51, viewed by focusing the eye for a 
a distance, or by the stereoscope, represents a cone, with 
the small hexagon distant ; but by squinting, the small end 
is made to stand ont towards the observer. As the former 
is always the way of viewing objects by the stereoscope, it is 
only necessary here to explain how the sensation of depth and 
distance is produced by distant focusing, or viewing each half 
with the eye of the same side, and for the sake of simplicity 




Fig. 53. 

only four points in each figure, A, C, D, B, and A, C, D, B, 
need be taken for the purpose of illustrating the mode in which 
the eye has to unite every corresponding pair of points in 
turn. In the diagram. Fig. 52, these letters represent the same 
points of the hexagons, as in Fig. 51. 

The points A and A, which correspond in the two larger 
hexagons, are united when the right optic axis R ^ is directed 
to A, and the left L A to A ; but the resulting single point 
appears to be at E, where these axes meet ; so, in like manner, 
B and £ unite, and form one point apparently, at F. The 
points C C of the smaller hexagons being more distant from 
each other than A from A, or B from B, can only be united 
by a more distant prolongation of the optic axes L C and R 
C to the point of union G, which thus appears to be the posi- 
tion of these points of the smaller hexagons, and so with re- 
gard to D and D ; and as these points are merely taken as 
samples of the whole, the result is that the corresponding parts 
of the lesser hexagons being more distant from each other than 



THE 8TERE6SC0PE. 35Y 

those of the greater hexagons, the former, when united, appear 
more distant than the latter, and the figure stands out from the 
paper. 

The stereoscope is merely an instrument to cause a super- 
position of the right and left views ; but this alone is not suf- 
ficient to produce a stereoscopic effect, as the diagram Fig. 53, 
will show, in which the two halves of Fig. 51 are represented 
superposed, the two outer hexagons coincide, but the similar 
parts of the two inner need still to be brought into union in 
the manner represented in Fig. 52, and the various distances 
at which the optic axes meet appear to the mind to be the dis- 
tance of the objects represented. 

In Mr. Wheatstone's reflecting stereoscope, mirrors are 
used. The principle of the instrument is as follows : — Objects 
placed in front of a mirror have their reflected images appar- 
ently behind the mirror. By arranging two mirrors at a cer- 




Fig. 54. 

tain inclination to each other, the images of the double picture 
may be made to approach until they coalesce, and the eye per- 
ceives a single one only. The preceding diagram will explain 
this. 

The rays proceeding from the star on either side pass in the 
direction of the arrows, being thrown off from the mirror 
(represented by the thick black line) and entering the eyes at 
R and L. The reflected images appear to the mind behind 
the mirror, uniting at the point A, where the optic axes 
meet. 

The reflecting stereoscope is adapted principally for viewing 
large pictures. It is a very perfect instrument, and admits of 
a variety of adjustments, by which the apparent size and 
distance of the stereoscopic image may be varied almost at 
pleasure. 



358 



BINOCULAR PHOTOGKAPHY. 



The refracting stereoscope is a more portable form of appa- 
ratus. A sectional view of the common form is given in the 
diagram (Fig. 55). 

The brass tubes to which the eyes of the observer are ap- 
plied contain each a semi-lens, formed by dividing a common 
lens through the centre and cutting each half into a circular 




Fig. 55. 

form (Fig. 56). The half lens viewed in section (Fig. 56) is 
therefore of a prismatic shape, and when placed with its sharp 
edge as in the diagram (Fig. 55), alters the direction of the 
rays of light proceeding from the picture, bending them out- 
wards or away from the centre, so that, in accordance with the 
law previously mentioned, the mind refers them to the direc- 
tion of the prolonged optic axes — represented for one pair of 




Fig. 56. 

points by dotted lines (Fig. 55), and the corresponding parts 
appear to be at the various distances, at which the optic axes, 
directed to them, meet. In the instrument as it is often sold, 
one of the lenses is made movable, and by turning it round 
with the finger and thumb, it will be seen that the positions 
of the images may be shifted at pleasure. 



t;he stekeoscope. 359 

Stereoscopes are now often mounted with whole lenses in 
place of the semi-lenses above described. In that case, the 
images coalesce by an involuntary action of the eyes of the 
observer. If, indeed, the whole lenses were set wider apart 
than 2^ inches, it might be conjectured that vision would take 
place principally through the inside edges, and that the same 
effect as that of a semi-lens would be produced. A single ex- 
periment, however, will suffice to show that even when the 
centres of the lenses are exactly opposite to the eyes, the two 
pictures combine without any effort. 

Rules for taking Binocular Photographs. — In viewing 
very distant objects with the eyes, the images formed on the 
retinas are not sufficiently dissimilar to produce a very stereo- 
scopic effect ; hence it is often required in taking binocular 
pictures, to separate the cameras more widely than the two 
eyes are separated, in order to give a sufficient appearance of 
relief. Mr. Wheatstone's original directions were to allow 
about one foot of separation for each twenty-five feet of dis- 
tance ; but considerable latitude may be permitted. 

If the cameras are not placed far enough apart the dimen- 
sions of the stereoscopic image from before backwards will be 
too small — statues looking like bas-reliefs, and the circular 
trunks of trees appearing oval, with the long diameter trans- 
verse. On the other hand, when the separation is too 
wide, the reverse obtains — objc-ts, for instance, which are 
square, assuming an oblong shape, pointing towards the ob- 
server. 

To understand the cause of this, it is necessary to refer to 
Fig. 52, where it was shown that those correspotiding parts in 
the two halves of a stereoscopic picture which are most widely 
separated, appear most distant when united in the st^ereoscope, 
and hence the greater the separation, the greater is the ap- 
parent distance of the object; but the effect of separating the 
cameras too widely is to produce too great separation of the 
distant parts of an object, as a little refiection on Fig. 51 will 
show ; it is evident that if the eyes, or the cameras which re- 
present them, were separated further in viewing the hollow 
cone, more of the sides D B and A C would be seen, and con- 
sequently the smaller hexagons would be placed more apart, 
and so when the two images were united in the stereoscope, 
the cone would appear longer. 

The effect is most observable when the picture embraces a 
variety of objects, situated in different planes. In the case of 
views which are quite distant, no near objects being admitted. 



360 BINOCULAR PHOTOGKAPHY. 

the cameras may be placed with especial reference to 
them, even as far as twelve feet apart, without producing dis- 
tortion. 

It is sometimes observable, in looking at stereoscopic pic- 
tures, that they convey an erroneous impression of the real 
size and distance of the object. For instance, in using the 
large reflecting stereoscope, if, when the adjustments have 
been made, and the images properly united, the two pictures 
be moved slowly forward, the eyes remaining fixed upon the 
mirrors, the stereoscopic image will gradually change its 
character, the various objects it embraces appearing to become 
diminished in size, and approaching nearer to the observer ; 
whilst if the picture be pushed backwards, the image will en- 
large, and recede to a distance. So, again, if an ordinary slide 
for the lenticular stereoscope be divided in the centre, and — 
looking into the instrument until the images coalesce — the 
two halves be slowly separated from each other, the 
solid picture will seem to become larger, and to recede from 
the eyes. 

It is easy to understand the cause of this. When the pic- 
tures in the reflecting stereoscope are moved forwards, the 
convergence of the optic axes is increased ; the image there- 
fore appears nearer, in accordance with the last-mentioned law. 
But to convey the impression of proximity to an object is 
equivalent to an apparent diminution in its size, for we judge 
of the dimensions of a body very much in relation to its sup- 
posed distance. Of two figures in bronze or marble, for in- 
stance, appearing of the same height, one known to be a 
hundred yards off might be considered colossal, while 
the other, obviously near at hand, would be viewed as a 
statuette. 

The most convenient camera for stereoscopic pictures is one 
for a plate 8 by 5 inches. It must be fitted with a pair of 
precisely similar lenses, either portrait or landscape objectives, 
according to the nature of the subject. The camera must 
have a partition to confine the rays from each lens to its own 
division. This is equivalent to two cameras placed side by side. 
The lenses may be mounted about 3^ inches apart. 



CHAPTER XXiy. 

CHEOMOTYPE OR LAMBERTYPB. 

In the article on carbon printing, in another page of this 
book, we have described that process, with several improve- 
ments that have been effected since the principles involved in 
it have been carried out to a state of practical perfection. We 
have now to describe a carbon process of another kind, which, 
although not sensibly different from the other in its principles 
of action, is jet carried out in practice so differently in regard 
to its minutise, and refined as respects results, as to warrant its 
being considered as revolutionary. We allude to the chromo- 
type, or, as it is termed by some, the lambertype process. 

In this process, the picture instead of being developed upon 
a hard and unyielding surface, such as that of a zinc plate, is 
supported upon a film of somewhat porous collodion. This 
film is afterwards transferred with the print (and may be said 
to form part of the picture itself), the glass supporting it hav- 
ing previously been prepared to facilitate its removal. The 
advantage gained by the employment of the collodion is two- 
fold. It tends to hold together the more delicate portions of 
the picture during the development, and, by reason of the 
gloss conferred by the film, it gives great transparency to the 
shadows, and thereby overcomes one of the hitherto weak 
points in the carbon process — namely, that the pictures are 
heavy, and lack the brilliancy of those made in silver, which 
is the case when zinc plates or flexible support are used. 
Until the introduction of this modification, the carbon process 
could not be considered as one well adapted for the pro- 
duction of small work — such as the carte and cabinet size ; 
hence its application was principally confined to pictures of 
large dimensions. With this modification, however, we have 
the means of producing prints possessing either the surface of 
ordinary, or of doubly albumenized paper, or one equal to that 
of the so-called enamelled prints, now so much in vogue — and 
possessing equal transparency in the shadows. For the chro- 
motype process a special tissue is supplied, but its speciality 
consists in its color only — that of a rich purple-toned silver 
print. In its composition, it does not differ from the ordinary 



362 CHROMOTYPE OR LAMBERTYPE. 

portrait tissues, any of which will answer the purpose quite as 
well, so far as the process itself is concerned. With this ex- 
planation we will proceed to the details of the manipulation. 
Sensitizing the Tissues. — The strength of the bath should 
be governed by the temperature, as well as by the general 
character of the negatives to be printed from. The higher the 
temperature, the weaker should be the solution, and vice 
versa. It may be as well to mention here, that the stronger 
the bath is, the softer will be the resulting prints, and the 
weaker it is, the greater will be the contrasts ; hence, for nega- 
tives of a weak and delicate character a dilute bath should be 
employed, and for more vigorous or hard ones, a more concen- 
trated solution must be used in order to obtain the best results 
from them. A very suitable bath for general purposes is — 

Bichromate of potash, » 4 ounces. 

Water, 100 ounces. 

Liquor ammonia, 1 dram. 

For use in summer, or for negatives lacking in contrasts, the 
proportion of water should be increased to 120 ounces. In 
winter, or for negatives with extra vigor, it will be advisable 
to reduce the water to 80 ounces. The temperature of the 
bath should never exceed 50 deg. Fahr. in summer, or there 
will be a danger of the pigmented coating dissolving and run- 
ning off the paper when it is suspended to dry in a heated at- 
mosphere. SulBcient of the solution should be poured into a 
porcelain dish or zinc tray (the latter is usually employed), so 
as to be at least 1-| inches in depth. The tissue, cut to conve- 
nient sizes, is now immersed, and any adherent air bubbles 
quickly removed with a broad, flat camel's-hair brush. It 
should be kept in motion all the time it is in the solution, in 
order to secure uniformity in sensitiveness throughout the 
sheet. With regard to the time of immersion, no definite rule 
can be laid down, as so much must depend upon the tempera- 
ture, for if it be very cold the solution will be absorbed by the 
gelatine much slower than if it were warmer. The best guide 
for removal is when the tissue has become thoroughly pliable. 
The longer the tissue is allowed to remain in the solution the 
greater will be the quantity absorbed, and the more sensitive 
it will become ; hence it will be seen that, with the same solu- 
tion, it is quite possible to obtain various degrees of sensitive- 
ness, so as to suit different characters of negatives. The more 
sensitive the tissue is made, the shorter the time it will keep 
in good working condition. 



CHEOMOTYPE OR LAMBEKTTPE. 363 

When the tissue is deemed sufficiently sensitized it is slowly 
drawn out, face downward, over the edge of the dish — or bet- 
ter still, over a glass rod fixed upon it. This will remove a 
large proportion of the superfluous solution. It is now laid 
face upward, on a sheet of stout blotting hoard, which is then 
placed across a line or rail to dry A better plan than this, 
more particularly when the temperature is high, is, after re- 
moving the tissue from the solution, to place it, face down- 
ward, on a plate of glass, and then to pass a squeegee lightly 
over the back; by this means a greater quantity of the super- 
fluous solution is removed, and consequently there will be less 
risk of the coating running off in hot weather. Also, the tissue 
will dry more easily — a great advantage in damp weather ; and, 
fothermore, its surfaces will be more smooth and even, so that 
better contact with the negative can be secured in the print- 
ing. The sensitizing may be conducted in an ordinary room, 
as the tissue is practically insensible to white light while wet, 
and only acquires sensitiveness as it dries. 

The Drying of the Tissue. — This is a somewhat important 
part of the process ; indeed, it has been asserted by some, that 
if the tissue be properly dried, half the difficulties pertaining 
to carbon printing have been surmounted. At one period it 
was supposed that the tissue could not be dried too quickly, as 
by that means the greatest degree of solubility was secured. 
Further experience, however, has proved this to be a fallacy, 
and that excessive solubility is by no means desirable, viz. : — 
It is liable to cause reticulation, when the prints are developed 
on a collodion film ; the delicate half-tints cannot be so well 
preserved, and the tissue will prove very insensitive. On the 
contrary, if the time of drying be too protracted, then the tissue 
will be more or less insoluble, and the unacted upon portions of 
gelatine can only be dissolved with difficulty in the develop- 
ment. Also, it will be very sensitive, and quickly become 
totally insoluble by keeping. The best conditions of drying 
may generally be secured by suspending the tissue in a room 
where a fire has been burning for some hours and then allowed 
to die out. If sensitized over night, it should be quite dry by 
the morning. As a rule, carbon tissne that has occupied from 
six to nine hours in drying will usually be found in the best 
working condition. When dry, it should be carefully pre- 
served from the influence of the atmosphere in a metal case. It 
will then keep in good working order for many days. The 
author has kept it, under exceptionally favorable circumstan- 
ces, for several weeks without deterioration. The time of ex- 



364 CHKOMOTYPE OE LAMBEKTyPE. 

posnre in printing is judged in the same manner as described 
in the chapter on carbon printing. Chromotypes, wlien mount- 
ed with the highest gloss, are usually printed with an orna- 
mental border, bearing an imprint of the name of the artist, 
etc. This is accomplished by double printing. There are 
several ingenious contrivances for enabling accurate registra- 
tion to be secured, but it would manifestly be out of place to 
describe them in a work of this character. 

After the pictures are printed, they should be developed as 
soon as convenient, or there will be a risk of their becoming 
darker, as the action of light, once started, progresses even 
when the prints are kept in darkness. This property in the 
carbon process is termed the "continuating action of light," and 
is materially accelerated if the prints be kept in a moist and 
warm atmosphere — and practically arrested if they be made 
thoroughly dry, and preserved in a cool place. Carbon printers 
take great advantage of this property for securing a larger 
number of prints in a given time — for by printing only half, 
or even to a third of the requisite depth, and keeping them in 
the dark for from twelve to twenty-four hours, according to 
the state of the atmosphere, " fully exposed " impressions can 
be secured. 

Developinent of the Prints. — In the first place, some glass 
plates are required, of a convenient size for the work in hand. 
It is customary to develop several prints on the same plate, in- 
stead of having separate plates for each print. A glass 15 
inches by 12 is well suited for five cabinet-size pictures, when 
printed with the masked border. Beginners in the chromo- 
type process are usually advised to employ opal glass, of the 
kind known as "patent plate opal" — as, by reason of its 
white surface, the development can be more easily judged than 
when transparent glass is used. Beyond this, the opal glass 
ofiTers no advantage over the ordinary, and it is very expensive, 
its surface is easily scratched, and all scratches on the glass 
will be apparent in the finished picture. Whichever kind of 
glass be employed, the treatment is the same — its surface must 
be prepared to enable the collodion film carrying the picture 
to be afterwards transferred. There are two methods of ac- 
complishing this. One is to rub the surface over with pow- 
dered talc, the other is to give the glass a slight coating of wax. 
Both methods have their advocates, but the writer prefers the 
latter, notwithstanding that it involves a little more trouble. 

The waxing solution is made by adding 1^ drams of pure 
yeUow bqes-wax to a pint of benzole. This should be allowed 



CfiEOMOTYPE OB LAMBEKTYPE. 365 

to digest for a day or two with occasional agitation ; it is then 
allowed to subside, afterwards the clear portion is decanted for 
use. A little of this sohition is poured on to the plate, and 
evenly distributed with a piece of soft rag, or a pledget of 
" papier Joseph." After the benzole has evaporated, the glass 
is carefully polished with a piece of soft flannel, until all 
streaks are removed, and its surface appears clean and brilliant. 
The plate is then coated with collodion, of good body, made 
with pyroxyline of a somewhat porous character — about 5 or 6 
grains to each ounce of solvent is a good proportion to em- 
ploy. After the collodion has been allowed to set, rather more 
than is customary in the negative process, the plate is plunged 
into clean cold water and the ether and alcohol thoroughly 
washed out ; the plate is then ready for immediate use, or it 
may be kept in tl.'e water until required. 

To Mount the Prints for Development. — Place the col- 
lodionized glass on a smooth flat board, and immerse a suf- 
hcient number of prints to cover its surface in a dish of clean 
cold water. Remove all air bells, and allow the prints to re- 
main until they become quite limp, but not long enough for 
them to curl up gelatine side outward. Xow flood the plate 
with water, and place them in position upon it. When all are 
arranged, a piece of india-rubber cloth, somewhat larger than 
the glass, is laid, rubber side outward, over them, and a squee- 
gee applied as described in the chapter on carbon printing. 
The cloth is now removed, and the glass, with the adherent 
prints upon it, is placed between sheets of blotting paper for 
from live to ten minutes. The object of the india-rubber cloth 
is to prevent the collodion film from being abraded with the 
squeegee, and afterwards washing up during the development. 
The plate is now placed in water at a temperature of 90 deg. 
Fahr. After the lapse of a minute or two, the coloring matter 
will be seen to exude from the edges of the prints. "When this 
occurs one corner is carefully raised, so as not to injure the 
collodion, and the paper lifted off. The development of the 
prints is then conducted as described in the chapter on carbon 
printing. Should it be found that the pictures are over- 
printed, the temperature of the water may be increased some- 
what. If, on the contrary, they should show signs of under-ex- 
posure, the development must be completed with cooler water. 
^^ varying the temperature of the developing water any error 
in the exposure, within reasonable limits, may be compensated 
for. , 

When the development is completed, the plate is well rinsed 
with cold water, and then plunged into a solution of alum — 5 



366 CHKOMOTYPE OE LAMBfiRTYPE. 

ounces in a gallon of water — where it is allowed to remain for 
about five minutes, It is then well rinsed under the tap, and 
reared up to dry in a place free from dust. The action of the 
alum is to render the gelatine, which up to this time is partially 
soluble, insoluble, so that it can no longer be dissolved with 
hot water — the film in fact being practically converted into 
leather. 

If the prints are to be mounted with the full gloss — the ''en- 
amelled" surface — the spotting must be done before the 
transfer paper is applied. For this purpose, oil colors thinned 
with spirit of turpentine are used ; they will be dry in an hour 
or so after they are applied. The eliect of the pictures may 
sometimes be enhanced by strengthening the shadows, or soft- 
ening the lights, with powder colors applied with a wash-leather 
stump. 

To Transfer the Prints. — A sheet of double transfer paper, 
by preference of a slightly blue shade, a trifle smaller than the 
glass plate, is placed in warm water (100 deg. Fahr.), until its 
prepared surface acquires a '' slimy" condition. It is then re- 
moved and laid on the prints, which have previously been wet- 
ted with water, and the squeegee brought into use so as to 
ex*i3el all air bubbles and as much of the water as possible. The 
plate is then placed between blotting paper for a short time, to 
absorb the superfluous moisture. If the prints are required 
with the surface of doubly albumenized paper the prints are 
allowed to dry ; they are then stripped off and mounted with 
starch paste, and then rolled in the ordinary manner. If less 
gloss still be required, the prints, after stripping, are placed 
between sheets of damp blotting paper for an hour or so, and 
then mounted while they are still moist. 

But if the highest possible gloss be desired, the mounts 
must be attached before the prints are removed from the glass, 
which is done in the following manner : When the back of 
the transfer paper has become surface dry, and no more, the 
plate is held up to the light, and the opposite (diagonally) 
corners of each print is marked with a pencil. These marks 
are to serve as a guide for adjusting the mounts in the proper 
position. When all are marked, the back of the transfer 
paper is coated with thick starch paste, applied with a sponge, 
or thin glue ; the writer prefers the latter. The cards, which 
have previously been damped, are also coated with the 
cement, and then placed in position. When all are arranged, 
the plate is put under pressure between blotting paper for an 
hour or two, and then reared up to dry. In order to prevent 



Chkomotype ok lambektype. 367 

the prints springing ofi the glass before they are quite dry, 
which would cause an inequality in the gloss, American clips 
should be put round the edges. 

When the prints are perfectly dry, and not before, they are 
stripped off, and afterwards trimmed round the edges with a 
pair of sharp scissors. In place of mounting the prints on 
separate cards, it is customary with many to employ one sheet 
of card-board the same size as the glass, and to cut it up after- 
wards. Sometimes, instead of mounting the prints direct on 
to the mounts, a second or third sheet of transfer paper is 
applied, and when the whole is dry, the prints are stripped 
off and trimmed ; they are then mounted on the cards. By 
this means, considerable trouble is saved, as it is much easier 
to apply an extra sheet or two of transfer paper than the card- 
board mounts, and the additional thickness of paper prevents 
the mounting material from injuring the gloss of the picture. 




APPENDIX. 



HOW TO ASCERTAIN THE EQUIVALENT FOCUS OF A LENS. 

By the " equivalent focns" is meant that measured from 
the optical center of the combination, and which yields an 
image of a size corresponding with that obtained by using a 
thin simple lens. The knowledge of this is requisite for copy- 
ing, enlarging, and other purposes. 

An easy, although not absolutely accurate, method consists 
in focusing an engraving, or other well-defined object, as 
sharply as possible, having previously adjusted its distance 
from the camera so that the image on the ground-glass shall 
be of precisely similar dimensions to the original. Measure 
the distance between the gromid glass and the printed matter, 
and one-fourth of that measurement is the equivalent focus of 
of the lens. 

By some the equivalent focus is measured from midway 
between the lenses in the case of portrait and symmetrical 
combinations, or from the curved surface of a single achro- 
matic landscape lens ; but such measurements will yield re- 
sults which are only approximately accurate. The method 
first described by the late Mr. Thomas Grubb is one by which 
perfect accuracy may be obtained. Place the camera upon a 
level table on a sheet of paper fastened down upon the lat- 
ter ; then, having ascertained by inspection the limits of the 
subjects delineated upon the focusing screen, turn the camera 
round until the object at one extreme occupies the center of 
the ground glass, and accurately coincide's with a pencil line 
made thereon. JMow, with a pencil draw a line on the paper 
beneath the camera, parallel with one side of the latter ; turn 
the camera round until the object at the other extremity of 
the subject coincides with the pencil line on the focusing 
screen, and draw another line on the paper as before. The 
angle formed by the two lines thus drawn will show the 
"angle of picture" included. But this is not all ; for if we 
draw a line opposite to the angle thus formed, at such a dis- 



APPENDIX. 36d 

tance therefrom"as'sliall allow the third line, bounded by the 
two previous^ones/^to^be of the same length as the focusing 
screen, a perpendicular let fall from the angle upon the third 
line will give the equivalent focal length of the lens employed. 
Tills equivalent focal distance may or may not agree with the 
distance of the screen from the lens — most probably not, es- 
pecially if it be a double combination. 

To simplify this matter, we append the following diagram : 




Fig. 57. 

Let A B represent a moderatelj' distant view, and C the 
place of the camera. On focusing, we find the portion in- 
cluded on the ground glass to be from D to E only ; this is 
the angle of picture. If we now turn the camera towards D, 
so as to make its image fall on a line in the center of the 
focusing screen, and draw a line parallel with the side of the 
camera, we shall have a line parallel to C D. viz., c d. On 
turning the camera towards E, and making its image fall on 
the same mark on the screen, another line drawn parallel with 
the same side of the camera will also be parallel with C E, 
viz., G e, and these two lines are inclined to one another ex- 
actly in the same ratio as are C D and C E. 

By measurement upon the ground glass, we find the dis- 
tance between the images of the objects D and E to be equal 
to g h • and if we now place this line exactly opposite the an- 
gle G, SO as to be bounded by the lines g d, g e, and let fall the 
perpendicular <?/", the distance c/" is the equivalent focus of the 
lens. Any one possessing a lens and camera need therefore be 
under no uncertainty about the equivalent focus of the former ; 
and this once ascertained, he can always calculate the angle of 



370 APPENDIX. 

any view he may take with it, no matter upon what sized 
plate he may operate. 

QUANTITATIVE TESTING OF SOLUTIONS OF NITRATE OF SILVER. 

The amount of nitrate of silver contained in solutions of 
tliat salt may be estimated with sufficient delicacy for ordinary 
photographic operations by the following simple process : 

Take the pure crystallized chloride of sodium, — which oper- 
ative chemists make purposely for analysis by dissolving the 
best carbonate of soda in pure hydrochloric acid, — and either 
dry it strongly or fuse it at a moderate heat, in order to drive 
off any water which may be retained between the interstices 
of the crystals ; then dissolve in distilled water, in the propor- 
tion of 8|^ grains to 6 fluid ounces. 

In this way a standard solution of salt is formed, each dram 
of which (containing slightly more than one-sixth of a grain 
of salt) will precipitate half a grain of nitrate of silver. 

To use it, measure out accurately one dram of the bath in a 
minim measure, and place it in a two-ounce stoppered phial, 
taking caie to rinse out tlie measure with a dram of distilled 
water, and to add the rinsings to the fluid dram of bath ; then 
pour in the salt solutiou, in the proportion of a dram for every 
4 grains of nitrate known to have been present, in an ounce of 
the bath which is to be tested ; shake the contents of the bot- 
tle briskly, until the white curds have perfectly separated, and 
the supernatant liquid is clear and colorless ; then add fresh 
portions of the standard solution of salt by 30 minims at a 
time, with constant shaking. When the last addition causes 
no milkiness, read off the total number of drams employed 
(the last half dram being subtracted), and multiply that num- 
ber by 4 for the weight in grains of the nitrate of silver pres- 
ent in an ounce of the bath. 

In this manner the strength of the bath will be indicated 
within two grains to the ounce, or even to a single grain if the 
last additions of standard salt solution be made in portions of 
15 instead of 30 minims. 

Supposing the bath to be tested is thought to contain about 
28 grains of nitrate to the ounce, it will be convenient to be- 
gin by adding to the measured dram 6 drams of the standard 
solution ; afterwards, as the milkiness and precipitation become 
less marked, the process must be carried on more cautiously, 
and the bottle shaken violently for several minutes, in order to 
obtain a clear solution. A few drops of nitric acid added to 



APPENDIX. 371 

the nitrate of silver facilitate the deposition of the chloride ; 
but care must be taken that the sample of nitric acid employed 
is pure and free from chlorine, the presence of which would 
cause an error. The delicacy of this mode of testing may be 
increased by adding to the silver solution a little bichromate of 
potash, omitting the nitric acid ; a deep red precipitate of 
chromate of silver is thus formed, which tinges the chloride 
of silver formed on adding the standard solution, until the 
last, when all the silver having been precipitated, the chloride 
of sodium finally decomposes the red chromate of silver into 
the white chloride, and the completion of the operation is evi- 
dent from the change of color in the precipitate. 

The photographer may perhaps require to perform these 
operations when pure chloride of sodium is not immediately 
ol3tainable. In that case the ordinary commercial chloride of 
ammonium may be substituted, 7| grains being dissolved in 
6 fluid ounces of water. It is advisable, however, when using 
a chloride of doubtful purity, to take the precaution of trying 
the strength of the standard saline solution, by testing it upon 
30 grains of pure dried nitrate of silver dissolved in an ounce 
of water. 

SAVING OF SILVER AND GOLD FROM WASTE SOLUTIONS AND 

RESIDUES. 

The judicious collection of wastes from solutions of the noble 
metals is one of the best economies that a photographer can 
practice. Seeing that these substances are most expensive 
items which must be largely used, and also bearing in mind the 
fact that very little of them is really used up in forming the 
photographic image, it stands to reason that the rest can be re- 
covered. 

1st. A large glazed earthenware or porcelain jar is provided ; 
or an ordinary cask, tarred and dried inside, will answer. It 
should be fitted with a tap near the middle, to draw off super- 
fluous water when required. Into this jar are thrown all the 
drippings of nitrate of silver from sensitized paper, the first 
washing water of vessels which have contained the nitrate, the 
first washing water of photographic prints before toning, and 
in short all waste silver salts that can be made insoluble by a 
chloride or other haloid. 

The jar should be well supplied with chloride of sodium 
(common salt) to reduce the silver to chloride. But sometimes 
it happens that the latter insoluble compound is in a state of 



3t2 APPENDIX. 

such fine division that it does not readily subside so as to enable 
one to draw off, without waste, part of the superfluous liquid 
when necessary. In that case, acidulate with a little hydro- 
chloric or sulphuric acid, and stir the whole well up. In a 
short time the supernatant liquid will be quite clear and 
free from silver. 

2nd. Another saving of silver arises from the filters, the 
clippings of prints, and the bits of blotting-paper used for ab 
sorbing waste drops either in the negative or positive processes. 
These should all be dried and placed in a bag until they ac- 
cumulate in sufiicient quantity for reduction. To effect this 
the papers or other absorbents of nitrate of silver are burnt and 
the ashes carefully preserved. The burning may be done in an 
ordinary grate. First clear out all the ashes from the fireplace, 
fill the grate with the clippings, filters, etc., and set fire to them 
from the top. If ignition takes place from the bottom, the 
chances are that the draught will be immoderately strong, and 
waft up the chinmey some of the silver. Throw on all the 
papers, little by little, until they are reduced to a fine ash, care- 
fully sweep up the ashes, and either add them to the contents 
of the '• waste jar," or reserve them to mix with the precip- 
itated chloride after it has been dried. 

It should have been mentioned with respect to the ^ waste 
jar" that it should not be too highly charged with common 
salt ; because chloride of silver is, to some extent, soluble in 
concentrated solutions of chloride of sodium, and therefore a 
little waste might be incurred when drawing off the superfiu- 
ous liquid. Care also should be taken to notice that all the 
silver has been precipitated before drawing off the water. 
Tills is easily ascertained by adding a drop of solution of 
chloride of sodium to the liquid. If all the precious metal 
has not been precipitated a decided cloudiness will be ap- 
parent. 

3d. Another saving of silver arises from the fixing solutions 
of hyposulphite of soda. This economy is seldom practiced, 
for the reasons that the operation is a very disagreeable one. 
and as it is the custom now-a-days to use fresh solutions for 
every batch of prints fixed, the resulting proceeds are scarcely 
worth the trouble of recovering them. 

Silver is reduced from these used solutions by means of liver 
of sulphm' (sulphide of potassium). The form in which the 
silver is precipitated is that of an impure sulphide containing 
a great excess of sulphur. But this operation must be con- 
ducted at a distance from the dark room, else the sulphurous 



APPENDIX. 373 

fumes which are given off would be very prejudicial both to 
health, and more particularly to clean and clear photographic 
manipulation. 

4th. Recovery of Gold. — When the touing bath has appar- 
ently become exhausted and refuses to tone any more prints, it 
still contains a considerable quantity of gold which has become 
inert. To precipitate this, throw in a little of a solution of 
protosulphate of iron. The black deposit which is immediately 
formed consists mainly of carbonate and oxide of iron mixed 
with metalic gold in an extremely fine state of division-filter. 
The precipitate remains in the filter, which should be dried and 
burnt along with the silver filters. It is not worth while to 
keep these wastes separate, as the refiner allows for the gold 
when reducing the whole, and can estimate the value very 
accurately. 

When the time has come to clear the " waste jar " of its 
chloride or other insoluble salt of silver, all the clear water 
is drawn off with a syphon, the moist precipitate is spooned 
out into a large evaporating basin, or into one of those 
enamelled iron pots or pans which can now be purchased at 
most of the ironmongers' shops, and the water driven off by 
heat. 

All these " wastes," when diy, may be reduced together, and 
by any one who has competent metallurgical knowledge, and 
who is in possession of a furnace specially fitted for the work ; 
but from the writer's experience by far the best plan is to send 
the wastes to the professional refiner, who after recouping him- 
self for his trouble, will show more of the precious metal than 
one who only works occasionally can possibly do. 

REMOVAL OF SILVER STAINS FEOM THE HANDS, LINEN, ETC. 

The black stains upon the hands caused by nitrate of silver 
may be readily removed by rubbing them with a moistened 
lump of cyanide of potassium, leaving it on the hands for a 
little time, and then washing well with water. A solution of 
iodide of potassium allowed to dry on the hands will also, after 
a time, change the black stains into yellow iodide of silver, 
which may then be removed by hyposulphite of soda. 

Stains upon white linen may be easily removed by brushing 
them with a solution of iodine in iodide of potassium, and af- 
terwards washing with water and soaking in hyposulphite of 
soda or" cyanide of potassium, until the yellow iodide of silver 



3Y4: APPENDIX. 

dissolves out; the bichloride of mercury (neutral solution) 
also answers well in many cases, changing the dark spot to 
white. 

The following liquid, when other means fail, is an energetic 
remover of silver stains : Cyanide of potassium, 1 00 grains ; 
iodine, 10 grains; water, 1 ounce: the solution should be col- 
orless. 



MODE OF TAKING THE SPECIFIC GRAVITY OF LIQUIDS. 

Instruments are sold, termed " hydrometers," which indi- 
cate specific gravity by the extent to which a glass bulb con- 
taining air, and properly balanced, rises or sinks in the liquid ; 
but a more exact process is by the use of the specific gravity 
bottle. 

The bottles are made to contain exactly 1,000 grains of dis- 
tilled water, and with each is sold a brass weight, which coun- 
terbalances it when filled with pure water. 

In taking the specific gravity of a liquid, fill the bottle quite 
full and insert the stopper, which being pierced through by a 
fine capillary tube, allows the excess to escape. Then, having 
wiped the bottle quite dry, place it in the scale pan, and ascer- 
tain the number of grains required to produce equilibrium ; 
this number added to, or subtracted from, unity (the as- 
sumed specific gravity of water), will give the density of the 
liquid. 

Thus, supposing the bottle filled with rectified ether to 
require 250 grains to enable it to counterbalance the brass 
weight, — then 1. minus .250, or .750, is the specific gravity ; 
but in the case of oil of vitriol the bottle, when full, will be 
heavier than the counterpoise by perhaps 836 grains; there- 
fore 1 plus ,836, id est 1.836, is the density of the sample ex- 
amined. 

Sometimes the bottle is made to hold only 500 grains of dis- 
tilled water in place of 1,000 : in this case the number of grains 
to be added or subtracted must be multiplied by 2. 

The form of specific gravity bottle now most commonly used, 
has a brass weight which counterpoises the empty bottle only. 
This is an advantage, because the weight indicated by the 
liquid, when the bottle is filled, at onceindicates the specific 
gravity ; or in the case of a bottle holding 500 grains of dis- 
tilled water, the weight multiplied by 2 will indicate the sp. 
gr. The temperature should register 60 deg. Fahr. 



APPENDIX. 375 



REMOVAL OF VAENISH FROM GLASS PLATES, 

The black varnish may often be removed from collodion 
positives by means of chloroform or benzole, neither of which 
dissolves the pyroxyline constituting the film. 

To remove the varnish from a collodion negative apply a 
weak solution of caustic potash in water, to which a little 
alcohol is added. The precise strength of the solution is im- 
matej'ial, but the following may from long experience be rec- 
ommended ." 

Caustic potash, 1 ounce. 

Alcohol, 10 ounces. 

Water, • . 10 " 

After the varnish is dissolved, wash under a gentle stream of 
water. 

If the ultimate object be to detach the collodion film from 
the glass plate, it must next be placed in a 10 per cent, solu- 
tion of hydrochloric or acetic acid, care being taken not to 
damage the film when detached. 

If the utilization of the glass plate be the only object, the 
best way of removing old varnished films is to place the plate 
in boiling water, to which has been added common washing 
soda. This loosens the film in about half a minute. 

VARNISH FOR BLACKING- BKASS-WORK. 

Take the ordinary spirit-lacquer of the shops, and rub it up 
with vegetable black into a thin cream, afterwards filtering 
tlirongh muslin. The brass work must be heated before this 
varnish is applied. 

FORMULA FOR NEGATIVE VARNISH. 

Sandarac, 4 ounces. 

Alcohol, 28 " 

Oil of lavender, 3 " 

Chloroform, 5 drams. 

Another. 

Tough, hard, and durable : — 

Shellac, 1^ ounce. 

Mastic, 4- " 



3Y6 APPENDIX. 

Oil of turpentine, ....... ^ ounce. 

Sandarac, 1^ " 

Venice turpentine, ^ " 

Camphor, 10 grains. 

Alcohol, 20 fluid ounces. 

Another. 

Sandarac, 90 ounces. 

Turpentine, . ' 36 " 

Oil of lavender, 10 « 

Alcohol, 500 " 

Another. 

This one may be rubbed down with powdered resin, and 
gives a splendid surface for retouching: — 

Sandarac, 2 ounces. 

Seed Lac, 1 to 1-| " 

Castor oil, 3 drams. 

Oil of lavender, 1^ " 

Alcohol, 18 fluid ounces. 

NEGATIVE EETOUCHING VAKNISH, 

Sandarac, 1 ounce. 

Castor oil, 80 grains. 

Alcohol, 6 ounces. 

First dissolve the sandarac in the alcohol, and then add the 
oil. 

GROUND GLASS VARNISH. 

Sandarac, , . • 90 grains. 

Mastic, 20 " 

Ether, . . . ■ 2 ounces. 

Benzole, \\o\\ " 

The proportion of the benzole added determines the nature 
of the matt obtained. 

SOLUTION FOR SILVERING GLASS MIRRORS (Martin's). 

A. 

Nitrate of silver, 175 grains. 

Distilled water, 10 ounces. 



APPENDIX. 377 



B. 



Nitrate of ammonia, 262 grains. 

Distilled water, 10 ounces. 

C. 

Pure caustic potasli, . . 1 ounce (avoirdupois). 
Distilled water, ... 10 ounces. 

D. 

Pure sugar candy, . . ^ ounce (avoirdupois). 
Distilled water, ... 5 ounces. 

Dissolve and add — 

Tartaric acid, . . . . 50 grains. 
Boil in a flask for ten minutes, and when cold add — 

Alcohol, 1 ounce. 

Distilled water quant, suff. to make up to 10 ounces. 

For use take equal parts of A and B. Mix together also 
equal parts of C and D, and mix in another measure. Then 
mix both these mixtures together in the silvering vessel, and 
suspend the mirror face downward in the solution. 

INTENSIFYING SOLUTION FOE GELATINE NEGATIVES. 

(Edwards'.) 

Saturated solution of bichloride of mercury, 10 ounces. 
Iodide of potassium, 10 dram. 

Dissolve the iodide of potassium in ten ounces of water, 
and pour gradually into the mercurial solution until the pre- 
cipitate thrown down is nearly redissolved. Add one ounce 
of hyposulphite of soda in crystals. 

Another. 

Mercury bichloride, . 60 grains in 4 ounces water. 

Iodide of potassium, .90 " 2 " 

Hypo, 120 " 2 " Mix. 



3T8 APPENDIX. 



INTENSIFYING SOLUTION FOR GELATINE. 

(Mr. William England's.) 

Mercuric chloride, . 20 grains. 

Ammonium chloride, 20 " 

Water, 1 ounce. 

Wash the negative thoroughly after fixing, and appl}' the 
above until the film acquires a uniformly grey tint. Wash 
again, and apply a very weak solution of ammonia, 10 drops 
of the latter to an ounce of water. — 

TO EESTOEE FADED NEGATIVES. 

Mr. W. C. Debenham recommends the following solution 
for the purpose of restoring printing force to negatives which 
have faded after mercurial intensification : 

Schlippes' salt, . . . 10 grains. 

Water, . , . . . . 1 ounce. 

Wet the film thoroughly by soaking in a dish of water, and 
immerse in the restoring fluid until the desired effect is 
obtained. 

TO REMOVE THE LAST TRACES OF HYPO FROM THE FILM. 

Peroxide of hydrogen (20 vols.), ... 1 dram. 
Water, 5 ounces. 

After washing the negative well, it is immersed for a couple 
of minutes in the solution and again rinsed in water, when the 
intensification with silver can be at once proceeded with. 

Another. 

Where peroxide of hydrogen is not obtainable, the following 
may be used as a substitute, the solution containing that sub- 
stance in combination with others : — 

Barium dioxide, 1 ounce. 

Glacial acetic acid, 1 " 

Water, 4 " 

Reduce the barium dioxide to a fine powder, and add it 
gradually to the acid and water, shaking till dissolved. A 
few minutes' immersion in this solution will effectually remove 
or destroy the last traces of hypo. 



APPENDIX. 3Y9 

Another. 

A simple plan, brought forward by Captain Abney for this 
specific purpose, consists in employing a saturated solution of 
alum in place of the solution of hydroxyl or peroxide of 
hydrogen. 

EAU DE JAVELLE. 

Dry chloride of lime (hypochlorite of lime), . 2 ounces. 

Carbonate of potash, 4 " 

Water, 40 " 

Mix the chloride of lime with thirty ounces of water ; dis- 
solve the carbonate of potash in the remainder. Mix, boil, 
and filter. 

CO well's clearing solutions foe gelatine negatives. 



Alum, 2 ounces. 

Citric acid, L " 

Water, TO '' 

Wash moderately after fixing, and immerse the negative in 
the above. 

Another. 

Saturated solution of alum, 20 ounces. 

Hydrochloric acid (commercial), .... 1 " 

Immerse the negative after fixing, having previously washed 
it for two or three minutes under the tap ; wash well after re- 
moval from the alum and acid. 

WEIGHTS AND MEASUEES. 

Troy or ApotheGaries' Weight. 

1 Pound = 12 ounces. 1 ounce = 8 drams. 1 dram = 3 
scruples. 1 scruple = 20 grains. (I ounce troy = 480 grains, 
or == 1 ounce avordupois plus 42.5 grains). 

Avoirdupois Weight. 

1 pound ^16 ounces, 1 ounce = 16 drams. 1 dram = 
27.343 grains. (1 ounce = avoirdupois = 43T.5 grains), (1 
pound avoirdupois = 7,000 grains, or = 1 pound troy plus 2^ 
troy ounces plus 40 grains). 



380 



APPENDIX. 



Imperial Measure. 

1 gallon = 8 pints. 1 pint ^ 20 ounces. 1 ounce = 8 drams. 
1 dr&.ni = 60 minims. (A wine pint of water measures 16 
ounces, and weighs a pound.) 

An imperial gallon of water weighs 10 pounds avoirdupois, 
or 70,000 grains. An imperial pint of water weighs \\ pound 
avoirdupois. A fluid ounce of water weighs 1 ounce avoirdu- 
pois, or 437.5 grains. A fluid dram of water weighs 54.7 
grains. A minim weighs 0.91 grains. 

French Measures of Weight". 

1 kilogramme = 1,000 grammes = something less than 2^ 
pounds avoirdupois. 

1 gramme = 10 decigrammes = 100 centigrammes = 1,000 
milligrammes = 15.433 English grains. 

A gramme of water measures 1 cubic centimeter, or 17 
English minims, nearly. 1,000 grammes of water measure 35|- 
English fluid ounces. 

French Fluid Measures. 

The cubic centimeter usually represented by " c. c." is the 
unit of the FreDch measurement for liquids. It contains 
nearly seventeen minims of water ; in reality, it contains 
16.896 minims. The weight of this quantity of water is one 
gramme. Hence it will be seen that the cubic centimeter and 
the gramme bear to each other the same relation as our dram 
for solids and the dram for fluids, or as the minim and the 
grain. The following table will prove to be sufiiciently 
accurate for photographic purposes : 



1 cubic 


centimeter 


= 


17 minims 


(as near as possible). 




2 cubic centimeters 


= 


34 










3 




= 


51 










4 




= 


68 


orO 


ounce 


1 dram 


8 minims 


5 




— 


85 


" 


" 


1 " 


25 


6 




= 


102 


" 


" 


1 " 


42 


7 




= 


119 


" 


" 


1 " 


59 


8 




= 


136 


" 


" 


2 drams 16 


9 




= 


153 


" 


" 


2 " 


33 


10 




= 


170 


" 


" 


2 " 


50 


20 




= 


340 


" 


" 


5 " 


40 


30 




•= 


510 


" 1 


" 


" 


30 


40 




= 


680 


" 1 


" 


3 " 


20 


50 




= 


850 


" 1 


" 


6 " 


10 


60 




= 


1020 


" 2 


ounces 


1 dram 





70 




= 


1190 


" 2 


" 


3 drams 50 " 


80 




=^ 


1360 


" 2 


" 


6 " 


40 


90 




= 


1530 


" 3 


" 


1 dram 


30 


100 




= 


1700 


" 3 


" 


4 drams 20 



APPENDIX. 



381 



French Measures of Length. 

I meter = 10 decimeters = 100 centimeters = 1,000 milli- 
metres == 39.37079 English inches. 

A meter is equivalent to the ten-millionth part of the arc of 
the meridian, extending from the equator to the pole. 

The Conversion of French into English Weight. 

Although a gramme is equal to 15.4346 grains, the decimal 
is one which can never be used bj photographers ; hence in 
the following table it is assumed to be 15|- grains which is the 
nearest approach that can be made to practical accuracy : 



grains. 



1 gramme 


= 


15f grains. 






2 grammes 


= 


30| " 






3 




= 


461 " 






4 




= 


61f " 


or 1 dram 


If 


5 




= 


77 " 


" 1 " 


17 


6 




= 


92| " 


" 1 " 


32f 


7 




= 


107| " 


" 1 " 


m 


8 




= 


123i " 


" 2 drams 


3i 


9 




= 


138f " 


" 2 " 


18| 


10 




= 


154 


" 2 " 


34 


11 




= 


169f " 


" 2 " 


49f 


12 




= 


184f " 


" 3 " 


H 


13 




= 


200^ " 


" 3 " 


201 


14 




= 


215f " 


" 3 " 


35f 


15 




== 


231 


" 3 " 


51 


16 




= 


246f " 


" 4 " 


6f 


17 




= 


26U " 


<• 4 " 


21| 


18 




= 


277i " 


" 4 " 


37i 


19 




= 


292f " 


" 4 " 


52f 


20 




= 


308 


" 5 " 


8 


30 




= 


462 


" 7 " 


42 


40 




= 


616 


" 10 " 


16 


50 




= 


770 


" 12 " 


50 


60 




= 


924 


" 15 " 


24 


70 




= 


1078 


« 17 ., 


58 


80 




= 


1232 


" 20 " 


32 


90 




== 


1386 


" 23 " 


6 


100 ' 




=. 


1540 


" 25 " 


40 



INDEX. 



PAGE 

Abnormal development 123 

Acetic acid 32 

Acids 20 

Acids and salts 26 

Albumen 33 

Albumenized paper printing. . . .263 

Alcohol 35 

Methylated 37 

Alum 37 

American Rapid Lens 346 

Ammonia 38 

Bichromate of 39 

Carbonate of 39 

Nitrate of 40 

Ammonium sulphide 39 

Bromide 40 

Chloride 40 

Iodide 41 

Sulphocyanide 41 

Aniline 41 

Process 317 

Aqua regia 41 

Argentometer 266 

Arsenic, bromide of 41 

Atomic theory 23 

Barium chloride 42 

Baryta, nitrate of 42 

Bases .... 19 

Benzole. 42 

Binocular vision 352 

Bitumen 43 

Bleaching positives 166 

Bromides, testing purity of 43 

Bromine 43 

Cadmium 44 

Bromide 44 

Iodide 45 

Calcium bromide 45 

Chloride 45 

Iodide 46 

Camphor 46 

Canada balsam 46 

Caoutchouc 46 

Carbolic acid 47 



. PAGE 

Carbon enlargements 261 

Carbon printing 281 

Castor oil 47 

Cellulose, different forms of. . . .136 

Charcoal, animal 47 

Chemical action, various iodides 149 

Chemical changes 27 

Chlorine 47 

Chloroform 48 

Chromatic aberration 333 

Chromium 49 

Chromotype or Lambertype. . . .361 

Citric acid 49 

Collodion 130, 140 

Collodion negatives 167, 236 

Collodion positives 159, 234 

Collodion transfers 254 

Collotype printing 300 

Combination by weight, laws of 23 

Compounds and mixtures 19 

Cyanogen 50 

Dark room 219 

Defects in prints 274 

Developed images, nature of . . .121 

Developer for positives 164 

Development of invisible image 103 
Distortion, its nature and cure. 341 
Double decomposition 28 ' 

Eau de Javelle 379 

Elementary bodies, tables of . . . . 16 
Engineers' plans, Willis's pro- 
cess for 319 

Enlargements 250 

Enlarging by artificial light 259 

Equivalent focus, how to ascer- 
tain 368 

Ether 50 

Fading of prints 212 

Ferrotypes 159, 234 

Ferrous oxalate 58 

Fixing agents 124 

Fixing prints 207 

Foci of lenses 329,336 



INDEX. 



383 



Fused nitrate of silver for bath.. 157 

Gallic acid 51 

Gallic acid as a reducing agent. .104 

Gelatine 52 

Gelatine emulsion process 303 

Intensifying solution for.. .377 

Negatives, Cowell's clearing 
solutions for 379 

Negatives, intensifying solu- 
tion for 378 

Glass house 245 

Glycerine, 52 

Gold, chloride of 53 

Hyposulphite of 54 

Gums 54 

Historical sketch of photog- 
raphy 1 

Hydriodic 54 

Hydrobromic acid 54 

Hydrochloric acid 55 

Hydrogen, peroxide of 55 

Hydrosulphuric acid 56 

Hypo, to remove last traces of ..378 

Invisible image, nature of 109 

Development of Ill 

Produced by ozone 121 

Iodide of silver, causes affecting 

sensitiveness of 115 

Iodine 56 

Iodizing solutions 145, 221 

Iron, acetate of 57 

Ammonio citrate of 57 

Ammonia-sulphate of 58 

Oxalate of 58 

Protosulphate of 58, 106 

Protonitrate of .106 

Perchloride of 59 

Iodide of 59 

Iron salts as reducing agents. ..105 

Kaolin 60 

Lead, acetate of 60 

Lenses, various forms of 329 

Light, action of, on chloride of 

silver 97, 99 

Light, action of, on organic sil- 
ver compounds 97,99 

Light, nature of 325 

Properties of 326 

Lime, chloride of 60 

Lithium, iodide of 61 

Litmus 61 



PAGE 

Magnesium 62 

Mercury, chloride of 62 

Micro-photography 322 

Moser's researches on invisible 
images 117 

Naphtha 63 

Wood 37 

Negative developers 176 

Negative solutions, formulas 

for..... 221 

Negatives, to restore faded. . . .378 

Nitrate bath 223 

Nitrate bath, chemistry of 152 

Nitric acid 63 

Nitrogen 65 

Nitro-glucose 65 

Nitro-hydrochloric acid 65 

Nitro-sulphuric acid for prepar- 
ing pyroxyline 134 

Nomenclature of compounds. . . 24 

Op tics of photography 325 

Organic substances, chemistry 

of 29 

Matter 66 

Orthoscopic lens 343 

Outlines of general chemistry. . . 15 

Oxygen 66 

Ozone 67 

Phosphoric acid 67 

Photo-block printing 298 

Photo-crayons 258 

Photo-lithography 288 

Plain paper printing 262 

Platinotpye printing. ... 276 

Portrait lenses 348 

Portraiture 245 

Positive and negative photo- 
graphs 158 

Positive printing 184, 262 

Potash 68 

Bichromate of 68 

Carbonate of 69 

Nitrate of 69 

Oxalate of 71 

Permanganate of 71 

Potassium, bromide of 70 

Cyanide of 70,127 

Fluoride of 71 

Iodide of 71, 147 

Sulphide of 72 

Pyrogallic acid 73 

Pyroxylic spirit 73 

Pyroxyline 73 



384 



INDEX. 



PAGE 

Rapid rectilinear lens 348 

Reducing agents, action on in- 
visible image 103 

Reducing agents, substances em- 

plo3'ed as 104 

Refraction of light 326 

Removal of varnish from nega- 
tives 375 

Residues, saving of 371 

Salts 21 

Silver 73 

Acetate of 74 

Albuminate of 75 

Ammonio-nitrate of 75 

Bromide of 75 

Carbonate of 76 

Chloride of 76,93 

Citrate of 76 

Fluoride of 76 

Hyposulphite of 77 

Iodide of 77, 94 

Nitrate of 77, 91 

Nitrite of 79 

Oxide of 80, 95 

Sulphide of 80 

Silver salts, chemistry of . . . . 91 
Properties of 96 

Silvering glass mirrors, solution 
for 376 

Single landscape lens 338 

Toning and fixing bath 201 

Soda, acetate of 81 

Carbonate of 81 

Citrate of 81 

Hyposulphite of 82, 126 

Phosphate of 82 

Sulphite of 82 

Sodium, aurochloride of 82 

Chloride of 82 

Solar camera enlargements 254 

Specific gravity of liquids 374 

Spherical aberration 334 



PAGE 

Spontaneous decomposition in 

pyroxyline 140 

Stains, removal of 373 

Stannotj'pe, the 298 

Stereoscopic photography 352 

Storing negatives 244 

Sulphocyanides ,84 

Sulphuretted hydrogen 84 

Sulphuric acid 84 

Symbols of com^pounds 17 

Tannin 86 

Tartaric acid 86 

Testing permanence of prints. ..217 

Testing silver solutions 370 

Tetrathionic acid 87 

Toning by gold 201 

Toning by sulphur 200 

Transparencies in carbon 287 

Triple achromatic lens 344 

Turmeric 88 

Uranium, nitrate of 88 

Varnish, for blacking brass 

work 375 

Ground-glass 376 

Negative, formulas for 375 

Negative retouching 376 

Removal of, from glass 

plates 375 

Varnishing negatives 243 

Vocabulary of photographic 
chemicals 32 

Water 88 

Weights and measures 379 

Wet collodion, manipulations of.228 

Wide-angle lenses 347 

Wood blocks, printing on 287 

Woodburytype 295 

Zincography 291 



HOW TO MAKE 

PHOTOGRAPHS. 



r)TCTURE making is quite simple, and the details are there- 
l^ fore briefly given. Any person of average intelligence may 
feel certain that he can succeed in making good photographs if he 
purchases an equipment made by reliable manufacturers. 

Filling the Plate Holder. — If this is done in the daytime, 
a closet or room is selected and all white light excluded from it. 
It is a difficult task to make this exclusion absolute. One ray of 
white light will spoil a sensitive plate, and therefore the evening 
is generally chosen to develope negatives, and for illumination 




W. I. A. Improved Lantern. 

the light from a ruby lantern is employed. Open a package of 
gelatine plates (these plates are glass, with a coating of gelatine 
on one side) and place one of them in a Dry Plate Holder, with 
the sensitive (not the glossy) side facing outward. Handle the 
plates as shown in the outline cut. After putting into the holders 
as many plates as are needed for a day's work, pack the outfit so 
that it can be conveniently carried about. 



2 



HOW TO MAKE PHOTOGRAPHS. 



Taking the Picture. — For field service, a camera, a num- 
ber of plate holders filled with sensitive plates, a lens, tripod, 
carrying case, and focusing cloth are needed. When these have 
been taken to a place where the view looks 
inviting, fasten the camera on the tripod, throw 
the focusing cloth over your head, gather it 
under your chin, draw out the back of the 
camera, thus extending the bellows, and con- 
tinue the movement until 
the image on the ground 
glass appears most dis- 
tinct, then fasten the back 
of the camera. This is 
called "focusing." At the 

first glance an inexpe- VC^^V 

rienced person sees no 
reflection on the ground 
glass, but the eye soon 
becomes practiced to perceiving the inverted image there. Sub- 
stitute a plate holder for the ground glass, see that the cap is 




Amateur with Kit 
Packed. 




Plate in hand. 




Apparatus set up. 

on the lens, pull the slide out of the holder, and place it on the 
top of the camera, or in a convenient place. If everything is 
now in readiness, and the time for exposing the sensitive plate 



HOW TO MAKE PHOTOGRAPHS. 




determined, uncap the lens, recapping it at the end of the allotted 
time and replacing the slide in the holder. 

Make an entry on the Registering Slide of a similar import to this: 

After you have picture im- 
pressions on each sensitive 
film, rearrange your outfit in 
compact shape and return 
home. 

Making Negatives. — 
Amateurs may content them- 
selves with making the ex- 
posures and sending their Regristering si.de. 

plates in a light-tight negative box to some photographer, who, for 
a small price, will produce the finished pictures and mount them 
on card-board or in albums. 

It is not essential where one attends to these details him 
self, that they should be done at once. 
Months may elapse, and these Dry 
Plates be carried hundred of miles with- 
out deterioration. 

The Chemical Outfit for making neg- 
atives comprises the following articles ; 
Two glossy rubber trays, a 4 ounce 
glass graduate, a minim graduate, a 
ruby lantern, a bottle of S.P.C. negative 
varnish, 1 dozen dry plates, an ounce of 
bromide of ammonium, a pound of hypo- 
sulphite of soda, 1 pound of alum, and a package of S.P.C. car- 
bonate soda developer. , 
These chemicals are not dangerous nor do they emit offensive 
odors. Silver stains and the disagreeable smell of collodion 
belong to the old " wet " process. 

The exposed plates must be removed from the holder in a 
dark room illuminated by a feeble red or orange light. Take 
them out of the holder, carefully handling them by the edges and 
place one of them, film side up, in a tray of pure water*. While it 
soaks there prepare the developer in the following manner ; Dis- 
solve the contents of the paper package marked No. 3, of the 

* The Russell Negative Clasp and Drying Support for holding dry plates during de- 
velopment and drying is certainly to be recommended for convenience, and because it 
keeps the fingers out of the developing solutions. It also enables one to hold the plate 
up and note the progress of development without touching the sensitive surface of the 
plate. (See illustration on page 7). 




Negative Box. 



HOW TO MAKE PHOTOGRAPHS. 



carbonate soda developer in 64 ounces of water, and label 
this " Solution No. 2 ; " and with the minum glass add to 
2 ounces of this solution 2 drams of the No. 1 solution. Now 
pour off the water from the tray, and flow over the plate 
the combined developing solution. If 
air-bubbles form on the plate they must 
be removed by a touch of the finger or 
by a soft camel's hair brush. If the 
plate be correctly exposed traces of the 
image will appear on the sensitive film in 
a short time, but in case they do not, pour 
the developing solution back into the 
graduate and add a little more of No. 2 
solution (from a quarter to half an ounce) 
and reflow the plate with the strengthened 
developer. In a short time the details of 
the image will appear, but wait patiently 
until all the details are out and clearly 
seen in the deep shadows, and until the 
milky-white appearance of the plate is 
gray color. The negative is then fully 




Graduated Glass. 



changed to a dark 
developed and must be washed in two changes of water, when it 
is ready for the "fixing" bath. Should the image on the plate 
flash out suddenly on flowing it with the developing solution, and 
continue to grow very rapidly, the plate has been over-exposed 
and must be quickly removed from the developing tray and placed 
in pure water while a restraining solution of bromide is made as 
follows : Dissolve 1 ounce of bromide of ammonium in 8 ounces 
of water, and label "Bromide solution." Now add a few drops 
of the bromide solution to the developing tray and replace in it the 
partly developed plate. The development will now proceed more 
slowly, but if too much bromide has been added, so that the de- 
velopment is entirely stopped, it can be started again by adding 
carefully a little more of the No. 2 solution. 

In the unused tray mix a solution of 4 ounces of hyposulphite 
of soda and 20 ounces of water. Label this tray "Hypo." and 
do not use it for any other purpose. After washing the negative 
place it in the hypo, bath and allow it to remain there until 
every vestige of the milky-white appearance has vanished. The 
negative can now be safely examined by white light. It must be 
thoroughly washed, as the hyposulphite of soda, if allowed to 



HOW TO MAKE PHOTOGRAPHS. 



remain in the film, will crystalize and destroy the negative. A 
negative washing box will be found to be of great assistance. 
Meanwhile rinse out the first tray and partially fill it with a solu- 




Negative Washing Box. Negative Back. 

tion of 20 ounces of water and all the alum the water will hold in 
solution. Allow the plate to remain in the alum bath about five 




The Russell Negative Clasp. 

minutes and then thoroughly wash it again and set it on edge 
to dry in a negative rack or the drying support. 

All the preceding instructions can be briefly summarized as 
follows : 

1. Put some sensitive plates into dry plate holders. 

2. Make the exposures. 

3. After taking a plate out of the holder, place it in a tray 
filled with water. 



HOW TO MAKE PHOTOGRAPHS. 



4. Drain off the water and pour over the plate the mixed 
developing solution. 

5. Wash the plate and place it in the hypo, solution. 

6. Wash the plate and give it an alum bath. 

7. Wash the plate and set it in the rack to dry. 

8. When perfectly dry, coat the film over with negative 
varnish, and allow that coating to dry. After this the surface of 
the plate may be touched by the fingers. 

Making Prints from Negatives. — At this point the work 
ceases to be one of faith, as the results are now to appear. An 
outfit of printing requisites comprises a printing frame, a porcelain 
pan, a vulcanite tray, some ready sensitized paper, a bottle of 
French azotate, a bottle of chloride of gold, a glass graduate, 
some hyposulphite of soda, a glass form, a Robinson trimmer, 
some sheets of fine card-board, a jar of parlor paste, and a bris- 
tle brush. 

Blue Prints. — If you wish to make a blue picture on a 
white ground, commonly called a "blue print," procure a package 
of ferro-prussiate paper, place the negative, film side up, in a 
printing frame. Upon the negative lay a piece of ferro-prussiate 




Printing Frame. 

paper (this should be handled in a dim light) with the colored 
side down. Close the back of the printing frame and fasten 
it by setting the springs. Carry the printing frame to some 
place where the sunlight will fall upon it, and from time to 
time examine the print. As soon as the picture is clearly seen, 
take out the print and throw it into a pan containing clean water. 
After about twenty minutes remove the print and dry it in the 
sunlight. The result is a Dermanent blue and white picture, 



HOW TO xMAKE PHOTOGRAPHS. 



which will at least answer for a proof and show the merit of your 
negative. 

Sensitized Paper Prints. — In the morning prepare a ton- 
ing bath sufficient for the prints to be toned that day. Put 7^ 
grains of chloride of gold into 7^ ounces of water. Label the 
bottle, " Chloride of Gold Solution." Take 1 ounce of French 
azotate, 1^ ounces of the chloride of gold solution, and add 6 
ounces of water, and you have a toning bath which keeps well. 
Where the prints do not give the required tone, the bath must 
be strengthened by adding to it some new solution. Place the 
glossy side of a sheet of sensitized paper upon the film side of 
the negative in the printing frame. Do this in a very dim light. 

The printing has gone far enough when the print looks a little 
darker than you wish the finished picture to appear. Make as 
many prints from the negative as you desire. Wash the prints in 
several changes of water. Take seven ounces of the toning 
solution and change the prints to the pan containing it, where the 
prints should be turned over and over to make the toning even. 
The toning process should go on until the dark part of the pic- 
tures have a very faint purplish tint and the white portion is clear. 
Wash the picture, but preserve the toning solution. The pictures 
should now be left for twenty minutes in a solution composed of 4 




RobinBon Trimmer, 

ounces of hyposulphite of soda, 1 ounce of common salt, \ 
ounce of washing soda, and 32 ounces of water. This solution 
should also be prepared a day or two in advance. Give the pic- 
tures a final and effectual washing. After they are dried, lay 
them out one by one and, using the Robinson trimmer, cut 
them to the desired size. Now spread over the back of each 



HOW TO MAKE PHOTOGRAPHS. 



in turn some parlor paste, and lay them down with the center on 
the^sheets of card-board. This operation is called " Mounting 
Pictures." Press with a paper cutter upon the pictures and 
toward their edges until you are satisfied that they will lay flat. 

Further more explicit and complete instructions in the making 
of photographs, how they can be preserved in neat shape, in- 
structions for making stereoscopic and instantaneous pictures, 
transparencies, magic lantern slides, and photographs of micro- 
scopic objects, are to be found in a book which can be obtained 
for 50 cents per copy, published by the Scovill Manufacturing 
Company, entitled " How to Make Pictures," by Henry Clay Price, 




SCOVILL'S AMATEUR SPECIALTIES. 




THE 



SCOVILL 

Portable 

Dry Plate Qotfits 

FOR AMATETTES, 




Old Style E^uismest. 



ITew Sty £3a pa a^ 



•T^HE introduction of Dry Plates and the impetus given by them to the 
cause of Amateur Photography, created a demand for light and com- 
pact apparatus that could be easily carried about. That demand theScovill 
Manufacturing Company of New York anticipated and first met by the in- 
troduction of apparatus especially designed for the use of amateurs. 

When they announced an Outfit comprising a Camera, Holder, Tripod, 
Carrying Case, and a good Lens, for $10, a new era in Amateur Photog- 
raphy began, and it was destined to be henceforth a popular and cultivating 
recreation. 

The Cameras they make for amateurs are not mere toys — they have been 
used and approved by eminent photographers. Certainly no apparatus 
can compare with that made by our American Optical Co.'s Factory, in 
durability, accuracy and elegance of finish. It is in use in all parts of the 
globe, and has by merit won this wide-spread reputation. Be not deceived 
by what is copied after it. See that your apparatus bears the brand of their 
factory. 

Every article enumerated in this Catalogue has the guarantee of the 
Scovill Manufacturing Co., established in 1802, and well known throughout 
the world for fair and honorable dealing as well as for the marked 
superiority of their photographic apparatus and specialties. 

New Catalogues, circulars, etc., will be mailed to any 
one whose address is sent with the request for copies. 



10 SCOVILL'S AMATEUR SPECIALTIES. 



CATALOGUE 



OF 



-McBCOVILL'S*^ 




AMATEUR 

Photographic Keouisites. 



SCOVILL'S AMATElUR SPECIALTIES. 



11 



o4\^ 



FAVORITE 



<m^ 



All Articles of whicli are Warranted Accurate in Everj Respect. 

These Outfits are lighter, more compact, far handsomer and more accurate 

than any which are offered at the same price. Many professional 

photographers have bought them and use them constantly. 




In each outfit the Waterbury Lens, ^o which stops have recently 
been added, is worth the price of the whole outfit. 



FAVORITE OUTFIT A, Drice $10.00, comprises 




A Favorite VievV Camera with vertical shifting front, single swing 
movement, rubber bellows and folding platform \V\\\\ patent latch for making 
bed rigid instantaneous y, to produce 4x5 inch pictures, with 

1 Patent Double Dry Plate WoXd^iiX (^e.v&xi\\A&),W\\\\ patent Registering 

Slicies. 
1 Taylor Improved Folding Tripod. 

1 No. A "Waterbury" Achromatic Lens zuith a set of Stops. 
1 Carrying Case. 



12 



SCOVILL'S AMATEUR SPECIALTIES. 



FAVOEITE OUTFIT B, price $12.00, comprises 




A Favorite View Camera with vertical shifting front, single swing 
movement, rubber bellows and folding platform, with. patent latch for making 
bed rigid instantaneously, to produce pictures 5x8 inches ; also 

1 Patent Double Dry Plate Holder (Reversible), with /«^^«^ ^^^w/mw^ 

Slides, and with Kits. 
1 Taylor Improved Folding Tripod. 

1 No. B " Waterbury " Achromatic Lens with a set of Stops, 
1 Carrying Case. 



FAVOEITE OUTFIT 0, price $18.50, comprises 




A Favorite View Camera with vertical shifting front, single swing 
movement, rubber bellows and folding platform, with /a^^«^ latch for making 
bed rigid instantaneously, to produce 5x8 inch pictures. 

This Camera is constructed so as to make either a Picture on the full 
size of the plate (5 X 8 inches), or by substituting the extra front (supplied 
with the outfit) and using the pair of lenses of shorter focus, it is admirably 
adapted for taking stereoscopic negatives ; also, by the same arrangement, 
two small pictures, 4x5 inches each, of dissimilar objects can be made on 
the one plate. Included in this outfit are also 

1 Patent Double Dry Plate Holder (Reversible), with patent Registering 
Slides, and tvith Kits. 

1 Large " Waterbury " Achromatic Lens, with Stops. 

1 Pair "Waterbury" Achromatic Matched Stereoscopic Lenses, each 
with Stops. 

1 Taylor Improved Folding Tripod. 

1 Carrying Case. 



SCOVILL'S AMATEUR SPECIALTIES. 13 



FAVORITE OUTFIT D, price $14.00, comprises 

A Favorite View Camera with vertical shifting front, single swing 
movement, rubber bellows and folding platform, Wxxh. patent latch for making 
bed rigid instantaneously, to produce pictures 6^x8^ inches ; also 

1 Patent Double Dry Plate Holder (Reversible), with patent Registering 
Slides, and with Kits. 

1 Taylor Improved Folding Tripod. 

1 No, B " Waterbury " Achromatic Lens with a set of Stops. 

1 Carrying Case, 



FAVOEITE OUTFIT E, price $26.00, comprises 

A Favorite View Camera with vertical shifting front, single swing 
movement, rubber bellows and folding platform, -wxih patent latch for making- 
bed rigid instantaneously, to produce pictures 8x10 inches; also 

1 Patent Double Dry Plate Holder (Reversible), with patent Registering- 
Slides, and with Kits. 

1 Taylor Improved Folding Tripod. 

1 No. C " Waterbury" Achromatic Lens with a set of Stops. 

1 Carrying Case. 



EQUIPMENT A-A. 

Consists of Favorite Apparatus Outfit A, with 
1 Scovill Focusing Cloth. 
1 Dozen 4x5 Dry Plates. 
1 W. I. A. Improved Ruby Lantern. 

Complete for field service, Price, $13.25. 



EQUIPMENT B-B. 

Consisting of Favorite Apparatus Outfit B, with the additional articles 
enumerated in A-A. (Dry Plates 5x8 size.) 

Complete for field service. Price, $15.00. 



EQUIPMENT 0-0. 

Consisting of Favorite Apparatus Outfit C, with the additional articles- 
mentioned in Equipment A-A. (Dry Plates 5x8 size.) 
Complete for field service. Price, $31.50. 



EQUIPMENT D-D. 

Consisting of Favorite Apparatus Outfit D.with the additional articles 
enumerated in A-A. (Dry Plates 61 x 8i inches.) Price, $18.00. 

Where sensitive Plates are taken to a photographer's and there devel- 
oped, printed from, and mounted on card-board, any of the above Equip- 
ments lack nothing that is essential. We recommend the amateur to finish- 
his own pictures, and hence to procure one of the equipments on page 14. 



14 



SCOVILL'S AMATEUR SPECIALTIES. 



SCOVILL'S 



Pure Chemicals & Accessories 



FOR MAKING- NEG^ATIVES. 





. 1 



. __. ^_^_iz 



We offer for use with an}- Outfit to make' pictures 4x5 inches, the fol- 
lowing goods packed securely in a wooden case : 
1 pkg. S.P.C. Carbonate Soda De- , 1 lb. Alum, 



veloper, 
4x5 Glossy Rubber Pans, 
4 oz. Graduate. 
Minum Graduate, 
oz. Bromide Ammonium, 
lb. Hyposulphite Soda, 

PRICE, COMPLETE, $5.25. 



1 bot. S.P.C. Negative Varnish, 
1 doz. 4x5 Dry Plates, 
1 Scovill Focusing Cloth, . 
1 W. I. A. Ruby Lantern, 
1 Russell Negative Clasp. 



For use with any 5x8 Outfit we supply the same goods, with the 
exception of the substitution of 5x8 Pans and Plates for the 4x5 size. 

PRICE. 41^x51^ DEVELOPING OUTFIT, $5.50. 
"5x8 " " 6.50. 

♦* 6i^x8M " *• 7.00. 

*• 8xlO ** " 8.50. 



SCOVILL'S AMATEUR SPECIALTIES. 



15 



BIjiUK fribjts. 

S. P.C. 

Ferro-Prussiate Paper Outfit for Printing and Mounting 4x5 Blue 
Print Pictures. 



14x5 Printing Frame. 

1 4|x5i S.P.C. Vulcanite Pan. 
3 dozen" 4x5 S.P.C. Ferro-Prus- 
siate Paper. 

2 dozen sheets 6^x 8^ Card-board. 

Price complete, $2.80. Securely packed in a Wooden Box. 



1 Glass Form (for trimming prints). 
1 Robinson's Straight Trimmer. 
i Pint Jar Parlor Paste. 
1 1 inch Paste Brush. 



S.P.C. 

Ferro-Prussiate Paper Outfit for Printing and Mounting 5x8 Blue 

Print Pictures. 
This Outfit is like the one above, but with Printing Frame, Vulcanite 
Tray, Ferro-Prussiate Paper and Card-board adapted to 5x8 Pictures. 

Price complete, $3.50. Securely packed in a Wooden Box. 



6K X 8K Ferro-Prussiate Paper Outfit. Price, $4.25. 




s. p. 

Outfit for Printing, Toning, Fixing 
14x5 Printing Frame, 
15x7 Porcelain Pan Deep. 

1 4i X 5i S. P. C. Vulcanite Tray. 

2 dozen 4x5 S. P. C. Sensitized 

Albumen Paper. 
1 bottle French Azotate. ( For 
1 " Chlor.Gold,7igr. ( toning. 
1 2 ounce graduate. 



and Mounting 4x5 Pictures. 

1 lb. Hyposulphite of Soda. 

2 dozen sheets 6|x8| Card-board 

with Gilt Form. 
1 J Pint Jar Parlor Paste* 
1 1^ inch Bristle Brush. 
1 Glass Form (for trimming prints). 
1 Robinson's Straight Trimmer. 
Securely packed in a Wooden Box. 



16 SCOVILL'S AMATEUR SPECIALTIES. 

s. p. c. 

Outfit for Printing, Toning, Fixing and Mounting 5x8 Pictures. 
This outfit is like the one on preceding page, but with Printing 
Frame, Vulcanite Tray, Sensitized Paper, and Card-board adapted for 
6x8 Pictures. 

Price complete, $6.38. Securely packed in a Paper Box. 



4:i X 5^ Printing and Toning Outfit. Price, $5.00. 
6ix8i '' *' *' " 7.00. 

8x10 '' " " " 8.50. 



EQUIPMENT A-A-A. 

Complete in every Requisite for making the Highest Class Pictures. 

LACKING NOTHING FOR VIEW TAKING, DEVELOPMENT AND THE PRINTING 
AND MOUNTING OF PHOTOGRAPHS. 

Consisting of Favorite Apparatus Outfit A $10 00' 

Also 1 Developing Outfit 4x5 (see page 14."> 5 25' 

" 1 Sensitized Paper Outfit, 4x5 (see page 15.) 4 87 

Price, $20.00. 



EQUIPMENT B-B-B. 

Complete in every Requisite for making the Highest Class Pictures. 

Consisting of Favorite Apparatus Outfit B |12 OO 

Also 1 Z'^z',?/^/i«^ Outfit 5x8 (see page 14.) 6 50 

' 1 Sensitized Paper Outfit (see above) 6 38- 

Price, $24.50. 



EQUIPMENT C-C-C. 

Complete in every Requisite for making the Highest Class Pictures. 

Consisting of Favorite Apparatus Outfit C $18 50' 

Also 1 Developing Outfit 5x8 (see page 14.) 6 50' 

" 1 Sensitized Paper Outfit (see above.) 6 38' 

Price, $31.00. 

EQUIPMENT D-D-D. 

Consisting oi Favorite Apparatus Outfit D $14 00' 

Also 1 Developing Outfit (see page 14.) 7_00 

" 1 Sensitized Paper Outfit (see above) 7_00' 

Price, $28.00. 



SCOVILL'S AMATEUR SPECIALTIES. 



17 




DRY PLATE OUTFITS^ 



INTRODUCED IN 1554. 



^j}::^************************* 




These outfits are unsurpassed in neatness, lightness, and com- 
pactness, yet there is no question about their durability or 
serviceable qualities. On this account they have found favor 
everywhere. Each one is supplied with a patent reversing 
attachment, which has been styled " the lightning reverser." 



New York Outfit 601, size 4^^x53^, consisting of 

1 Finely Finished Single Swing Camera, with Folding Bed and 

Improved Dry Plate Holder, with Kits. 
1 No. 1 Extension Tripod, with Patent Reversing Attachment. 
1 No. A Waterbury Lens, with a set of Stops, and 
1 Compact Carrying Case, with Handle. Price, $12.00. 

New York Outfit 601 A, size 4^x63^, same as described above, 
except in respect to size. Price, $13.00. 

New York Outfit 602, size 5x8, same as described above, except 
in respect to size. Price, $15.00. 

New York Outfit 603, size 63^^x8^, same as described above, 
except in respect to size. Price, $18.00. 

New York Outfits not made larger than 63:^ x 83^ size. 



18 



SCOVILL'S AMATEUR SPECIALTIES. 



WATERBURY OUTFITS. 




The Waterbury Cameras which we introduced in 1885, are like 
other cameras and apparatus made by the American Optical Company — 
unapproachable ! 

For the benefit of such as have not seen a Waterbury Camera, we 
present the above illustration, and add that these cameras are made of 
mahogany. They have rubber bellows, folding platform, single swing, 
vertical shifting front, patent latch for making bed rigid instantaneousl)% 
and are as light and compact as substantia] cameras can be constructed. 

Fitted with 

Eastman-Walker 

Roll-Holder. 

New Model. 

4'x5 Waterbury Outfits, Complete $12 00 27 00 

CONSISTING OF 

1 Single Swing Camera, described above. 

1 New Style Double Dry Holder, with Patent Registering Slides. 

1 Wooden Carrying Case. 

1 Improved Taylor Tripod. 

1 No. A Waterbury Lens luith a set of Stops. 

5x8 Waterbury Outfits, Complete $16 50 36 50 

CONSISTING OF 

1 Single Swing Camera, described above. 

1 New St3de Double Dry Holder, with P.atent Registering Slides. 

1 Wooden Carrying Case. 

1 Improved Taylor Tripod. 

1 No. B Waterbury Lens with a set of Stops. 

6J4x8J^ Waterbury Outfits, Complete... .$20 00 44 00 

CONSISTING OF 

1 Single Swing Camera, described above. 

1 New Style Double Dry Holder, with Patent Registering Slides. 

1 Wood^ Carrying Case. 

1 Improved Taylor Tripod. 

1 No. B Waterbury Lens with a set of Stops. 

T i ii ( 4ix5i Waterbury Outfit, complete $14 00 

UUIUOI. ^5^r^ .. « .. jg ^jQ 



SCOVILL'S AMATEUR SPECIALTIES. 



19 



AMERICAN OPTICA L CO.'S APP ARATUS OUTFITS. 

This apparatus is manufactured in New York City under our immediate 
personal supervision ; and, as we employ only highly skilled workmen, 
and use nothing but the choicest selected materials, we do not hesitate to 
assert that the products of our factory are unequalled in durability, excel- 
lence of workmanship, and style of finish. This fact is now freely conceded 
not onl)'^ in this country but throughout Great Britain, Germany, Australia, 
South America, and the West Indies. 

OUTFIT No. 203, price $22.00, Consists of 
A Mahogany Polished Camera for taking pictures 4x5 inches, with 
Folding Bellows Body, single swing, hinged bed, and brass guides. It 
has a shifting front for adjusting the sky and foreground, with 

1 Daisy Double Dry Plate Holder, with Patent Registering Slides; also 

1 Canvas Carrying Case. 

1 Scovill Adjustable Tripod. 

OUTFIT No. 202 A, price $24.00, 

The same as No. 203, but with Camera for taking pictures 4^ x 5| inches. 
OUTFIT No. 202 B, price $26.00, for pictures 4irx64 inches. 
OUTFIT No. 203, price $30.00, Consists of' 

A Folding Mahogany Camera, well 
known as the '76 Camera (see illustra- 
tion). It is adapted for taking 5x8 
inch pictures, and also for stereo- 
scopic views — together with 

1 Daisy Double Dry Plate Holder, with 

Patent Registering Slids ; also 
1 Canvas Carrying Case. 
1 Scovill Adjustable Tripod. 
OUTFIT No. 204, price $42.00, Consists of 





A Folding Mahogany Camera of finest style and finish for taking 6i x 8i 
inch pictures, with 

1 Daisy Dry Plate Holder, with Patent Registering Slides; also 

1 Canvas Carrying Case. 

1 Scovill Extension Tripod, No. 3. 

For larger or special View Cameras, consult the American Optical 
Company's Catalogue. 

We recommend the purchase and use with the above Outfits of a 
Lens or Lenses selected from the list on page 32. 

,^t^f2l. Developing and Sensitized Paper Outfits to be used with the above, 
refer to pages 29 and 30. 



20 



SCOVILL'S AMATEUR SPECIALTIES. 



TOURISTS' POCKET OUTFITS. 





H 

A 

H 



H 




{Extract frottt Photographic Times, 
March, 1883.) 



American Optical Company's Tourists' 
Pocket Camera. 



TOURISTS POCKET CAMERA FOLDED. 



When folded up, a 4x5 Tourists' Camera measures but 5^x6^x2 
inches, and it is without any projecting parts, pins or screws, so that it may 
be slipped into and not tear a gentleman's pocket. The rods which are 
used to move forward the front of the camera are easily detached from it 
and drawn out of the bed. The connector at the other end of the rods is 
just as readily unset. To replace these three parts when the camera is 
brought out for service, requires no more time or skill than to take them off. 
They are nicely adjusted, and are polished and nickel plated, so that they 
add to the handsome appearance of the camera, and contrast well with its 
polished mahogany surface and the purple hue of its bellows. The weight 
of this camera and its dry plate holder (but \\ pounds for the 4x5 size) is 
on the center of the tripod. In focusing, the front of the camera and the 
lens are pushed forward, thus avoiding any disarrangement of the focusing 
cloth. When the focus is obtained, further movement of the lens is checked 
or stopped by means of a screw acting on a spring, which is pressed at the 
ends against the focusing rods." 




Tourist's Pocket Outfit No. 0206.-43:5 Tourist's Pocket Camera.^with 

1 Daisy Double Dry Plate Holder, with Patent Registering Slides. 
1 Scovill Extension Tripod No. 1, with patent reversing attachment. 
1 Canvas Carrying Case with Shoulder Strap. 

Price, complete, $22.00. 

Tourist's Pocket Outfit No. 0207.-5x8 Tourist's Pocket Camera, with 
1 Daisy Double Dry Plate Holder, with Patent Registering Slides. 
1 Scovill Extension Tripod No. 2, with patent reversing attachment. 
1 Canvas Carrying Case with Shoulder Strap. 

Price, complete, $30.00. 



We recommend the purchase and use with the above Outfits of a 
Lens or Lenses selected from the list on page 32. 

For Developing and Sensitized Paper Outfits to be used with the abovep 
refer to pages 29 and 30. 



SCOVILL'S AMATEUR SPECIALTIES. 



21 



SAINT LOUIS 
Reversible - Back Cameras. 

(patented.) 



TN addition to the desirable features which the Back Focus Reversible 
■*■ Camera possesses (see description below) the St. Louis Reversible- 
Back Cameras have the rack and pinion movement, patent latch' iox making 
the bed rigid instantaneously, and the ground-glass so arranged that the 
holder may be slid in front of it, as shown in the illustration. 

Each Camera is supplied with one Daisy Holder \v\\h. patent Registering 
Slides and canvas case. 




TITHE growing use of dry plates, and the desire for rapid exposures, 
''' led to the introduction of the American Optical Patented Reversible 
Back Cameras, and because they add to the grace and celerity of view- 
taking they have become vastly popular. A novel arrangement of a de- 
tachable carriage at the back combines such a multiplicity of adjustments 
in itself that a dry-plate holder may be reversed or be set for either an 8x10 
upright or horizontal picttire — all of these movements, without once changing 
the dry-plate holder in the carriage, which may be made to take an S. G. C, 
but not a Bonanza Holder. 

SAINT LOUIS REVERSIBLE-BACK CAMERAS. 

Fitted witli Eastman- Walker Roller Holder. 
New Model. 
Single Double Single Double 

For View. Swing-ijack. Swing-back. Swing-baek. Swing-back. 

4J^x53^ 126 00 $30 00 

5 x7 32 00 35 00 $52 00 $55 00 

61^x83^ 36 00 40 00 60 00 64 00 

8 xlO 40 00 44 00 70 00 74 00 

11 xl4 60 00 64 00 102 00 106 00 

Not made front focus above 11x14 size. 



22 



SCOVILL'S AMATEUR SPECIALTIES. 



Flammang's Patent Revolving-Back Cameras. 

(PATENTED.) 
Each Incased in a Canvas Bag, with Handle. 




"These are the finest View Cameras ever constructed," so says every 
photographer who has examined any of them, and this exclamation is not 
merely a tribute to the beauty and grace of their design, for invariably the 
desire has at the same time been expressed to possess one of these truly 
novel and substantial Cameras. 

Wherein lies the merit and attractiveness of the Revolving-Back Camera, 
that photographers want to cast aside cameras now in use and procure one 
of this new pattern? Briefly stated, it enables the view taker to secure 
either an upright or a horizontal picture without changing the plate holder 
after it has been slid into the carriage. No other camera can with such 
wondrous ease and celerity be changed from the vertical to the upright or 
vice versa. The carriage is simply turned about in the circle and automat- 
ically fastened. By this latter provision the carriage may be secured at 
either quarter of the circle. Ordinarily, the slide will be drawn out of the 
holder to the right ; but in certain confined situations, the ability to with- 
draw the slide to the left enables the photographer to obtain a view which 
he could not get wi-th the usual provision in a camera. The photog- 
rapher of experience is well aware of the difficulty, when taking an upright 
picture with a large camera wirhout the revolving back feature, of reaching 
up to draw out the slide at the top, and, what is more essential, of getting 
out the slide without fogging the plate in the holder. 

Grace and strength are combined in the Revolving-Back Camera, and 
its highly-desirable features are gained without the sacrifice of steadiness 
or any other essential principle in a good camera. Indeed, its merit is such 
that out-door photography has been advanced and made more attractive by 
its introduction. 

For a more detailed description consult Scovill's general catalogue. 



SCOVILL'S AMATEUR SPECIALTIES. 



23 



Revolving-back Camera, Front Focus. 

(patented.) 




PRICE LIST. 



Revolving-back Cameras, each incased in a canvas bag, with handle, 
and above 11x14 size, with two handles. 





REVERSIBLE. 


550A. For 
551- 


View 4 

4} 


X 


5 in. 


ix 


s%- 


551AB. ' 


4Mx 


(^% " 


551A. 


5 


X 


7 " 


551B. 


5 


X 


8 " 


552. 


6i^.x 


SK" 


553- 


8 


X 


10 " 


554. 


10 


X 


12 " 


555- 


II 


X 


14 " 


556 


14 


X 


17 " 


557- 


17 


X 


20 " 


557A. 


18 


X 


22 " 


558. 


20 


X 


24 " 


559- 


25 


X 


30 " 



single 
Smng. 



Double 
Swing. 



Fitted with Eastman- 

Walker Roll Holder 

New Model. 

Single Double 

Swing. Swing-back. 

I GO 



$46 00 



$31 GO $36 00 

.1/,. .1/ 11 33 00 38 GO 

.1/^ A1/ .< 24 00 39 00 

- ^ - " 35 00 40 GG 

- " "• " 35 00 40 00 

" Ai/^ 01/ " _ _ 45 GO 50 GO 

° " "-^ " 50 GO 55 00 

10 X 12 " 65 GO 70 GO 

" " - " 77 50 82 50 

"■■"""" .... 90 00 95 OG 

17 X 20 " 105 00 no GG 

" -^ - - . " _ _ J20 GO 130 GO 
25 X 30 " 165 GG 175 00 

These Cameras are fitted with Daisy Dry-plate Holders. 
Pleafee state, when ordering any size below 10x12, whether front or 
back focus is desired. 

Revolving-back Cameras with front focus not made above 8x10 size. 



55 00 


60 00 


55 00 


60 00 


69 CO 


74 00 


80 00 


85 00 


lOI GO 


106 GO 


119 50 


124 50 


140 GO 


145 GO 


170 00 


175 GO 


1S5 OG 


190 GO 


200 00 


210 GO 



24 



SCOVILL'S AMATEUR SPECIALTIES. 




THE SCOYILL MANIFOLD CAMERA. 

While 
this camera 
serves man- 
ifold pur- 
poses as its 
name indi- 
cates, noth- 
i n g could 
be more 
simple o r 
more easily 
m a n i p u - 
lated. The 
Manifold 
Camera has 
special advantages peculiar to itself and possesses the greatest number of 
desirable features which can be combined in a camera without sacrificing 
lightness and compactness, or having complicated adjustments. The 
unique device which controls the horizontal and vertical swings was 
patented by Mr. W. J. Stillman, of the editorial staff of the Photographic 
Times. To this has been added a ceittral latch for the purpose of bfitiging 
the swing movements within perfect control of the operator. An approximate 
focus is obtained quickl)' 
with the rear portion of 
the camera, which is pro- 
vided with the patent re- 
versible back. The fine 
focus is obtained by means 
of the rack and pinion 
movement, shifting the 
front upon which the lens 
is attached. 

While this camera is 
made to compass the great 
length of draw shown in 



the first illustration, the rear portion of the bed may be wholly detached' 
and when desired, one-third of the remaining portion of the platform ; a 
great advantage when photographing interiors, when an obtrusive tail board 
renders focusing almost an impossibility. With one-half of the bed taken 
off, this camera is still of the usual length of 
draw. The ground glass, when not in use, is dis- 
placed, not detached, by having the plate holder 
slid in front of it. This arrangement of ground 
glass and plate holder is shown in the second 
view. Still another noticeable feature is the 
absence of clamping screws from the front 
boards, to move which one needs but to press 
firmly against the lens. The bed folds in front 
of and behind the camera, and has the patented 
latch recent!}' devised at the American Optical 
Co.'s factor)^ PRICE LIST, including Canvas 
Case for Camera and one Holder, w'lih patent Reg. 
Slides. 

6i<x8i< size. ...$53 50 
SxlO sfze 58 00 





43^'x6i^ size..., $41 00 
5x7 size 42 00 



31^x41^ size. . . $34 00 

4x5 size 38 00 

41^x53^ size. ... 40 00 

Fitted with Eastman-Walking Roll Holder, New Model : 

4x5 size, $53 00 ; 4|x64. $.58 50 ; 5x7,^ $62 00 ; 6ix8i, |76 50 ; 8x10, $88 00 



SCOVILL'S AMATEUR SPECIALTIES. 25 




WITH WHICH TO SECURE MEMENTOES OF 
PLEASANT EXCURSIONS. 
So popular has amateur photography become among wheelmen that 
the two amusements are now often combined. The "Wheel" allows un- 
bounded opportunities to the amateur photographer to gather choice 
landscape views. 



"NE PLUS ULTRA" BICYCLISTS' PHOTO-OUTFIT 

(complete). 
PRICE, - - - $10.00. 

Consisting of a 3:^^x4^ Imitation Mahogan)- Camera with Vertical Shift- 
ing Front, Folding Bed, with patent latch. Double Dry Plate Holder, with 
tatent Registering Slides and Hinged Ground Glass. 

A UNIVERSAL JOINT BICYCLE ATTACHMENT, 

A No. A WATERBURY LENS, with Stops, 

A CANVAS BAG TO CARRY THE ABOVE, 7aith Shoulder Strap. 

The advantages of this outfit are its Lightness and Compactness, and 
the ease with which it can be brought into use — a new device on bed of the 
Camera permitting it to be made rigid, or to fold instantaneously. There 
are no loose pieces. The outfit complete weighs 2 pounds 3 ounces. 
NICKEL-PLATED BICYCLE ADJUSTABLE SUPPORT $1.50 

This has no loose pieces and is so accurately made as to have no 
side play. 

THE " MIGNON " BICYCLISTS' PHOTO-OUTFIT 

(complete). 

Consisting of a 8|^x4^ Finely Polished Mahogany Camera, with 
Swing Back, Vertical Shifting Front, Hinged Ground Glass, Folding 
Bed with Patent Latch, Rack and Pinion Movement (Front Focus). In 
improvement, and has no loose pieces. Nothing finer, short, it has every 
more attractive and yet simple was ever made. 

A Universal Joint Bicycle Attachment. 

A 5}4, inch Morrison Wide- Angle Instantaneous Lens, 
pronounced by authorities on optics to be without a peer. The Rotary 
Shutter with this Lens is the Most Compact and the Lightest known. 

A Canvas Saddle Bag lined with flannel to prevent marring of 
the fine finish of the camera. 



THIS OUTFIT COMPLETE WEIGHS LESS THAN TWO POUNDS. 



Price of "Mignon" Bicyclists' Photo-Outfit Complete, $70.00. 
Without Lens. $25.00. 



With the lenses just described, clear, sharp pictures can be obtained 
which will make fine transparencies and lantern slides, or which can be 
enlarged up to 8x10 size. 



26 



SCOVILL'S AMATEUR SPECIALTIES. 



SCOVIEL'S OUTFIT 

For PHOTOGRAPHING with the MICROSCOPE. 

Photographing with the microscope has hitherto been accomplished by 
the aid of elaborate and costly apparatus, and been applied chiefly to 
making illustrations for scientific magazines. The process used, that of 
wet collodion in connection with sunlight, involved the procurement of an 
expensive heliostat to produce a steady illumination, for with any less 
powerful light the exposure would necessarily be so prolonged that the 
coating of the plate would dry and become useless. Now all this is 
changed, for with the modern improvements in photography which are the 
result of the introduction of gelatinedry plates, the photographing of micro- 
scopic objects becomes as easy of accomplishment as the photographing of 
the beautiful and visible in nature is with the popular amateur outfits. 

The scientist and microscopist, instead of spending hours in making 
imperfect drawings, aided by the camera lucida, may in a few minutes, with 
the assistance of photography, produce a more perfect representation of a 
minute object than it is possible for the hand of man to do, working con- 
jointly with the eye. Not only can an enlarged image of a microscopic 
object be formed for illustration, but professors in colleges will find it a 
ready means to produce negatives of a suitable size from which may be 
made transparencies or magic lantern slides for exhibition to classes or the 
public. 




If this is done in the daj'time, a room from which all white light is ex- 
cluded should be selected ; but if used at night, as in most cases it would 
be, the operations may all be performed in the midst of a family group for 
their interest and amusement, and to impart to them knowledge of the mi- 
nute life or organisms of the world which the microscope alone can reveal. 



Scovill's Photomicroscopic Equipment, 



CONSISTING OF — 



Scovill Special Half Plate Camera. 
Multum in Parvo Lantern, with Double Condenser, 
dozen ^}4 x 6H size B Keystone Plates to make Negatives ; also 
dozen 3M x ^H size A Plates for Transparencies. 
Price, Complete, $18.00. 
The presumption is that you are provided with a microscope. If not, 
we recommend the purchase of one from a regular dealer in microscopical 
goods. 

Cifcular containing directions for use sent with each outfit. 



SCOVILL'S AMATEUR SPECIALTIES. 27 



MERCER PHOTOMICROGRAPHIC 



o; Jk. nvE :hi FL j^ 

Size, 2^x31^. 

Price, $6.50. 




This Camera is provided with a Brass Cone and Plate Holder 
with Ground Glass attached, to slide back and forth in the carriage, as 
desired. 



28 



SCOVILL'S AMATEUR SPECIALTIES. 



THE SCOVILL DETECTIVE CAMERA. 




It has not come to be generally known, but such is the fact, that Artists 
of renown and shrewd Detectives carry about these Cameras, and pictures 
are secured by them for their different lines of study through their instru- 
mentality in a manner which is perfectly simpl e — in fact, it requires no 
skill other than to get within the range of focus of the unsuspecting victim. 
As the party, whether man, woman, or child, is not aware that anything 
unusual is transpiring, the expression of the countenance and the pose are 
not arranged with reference to their appearance in a picture. A quick 
working lens is hidden in the camera, and also a few plate holders. By 
pressing on a spring the whole operation of exposure is completed. 

It followed naturally upon the introduction of the Roll Holder that it 
should be applied to the peerless SCOVILL DETECTIVE CAMERA, 
and this has been done in a manner that displays the greatest ingenuity. 
Instead of three double dry-plate holders, but one will accompany the 
Roll Holder. 
Scovill's Roll Holder Detective Camera, for 3^x4^ Pictures, with 

Instantaneous Lens $65 00 

Scovill's Roll Holder Detective Camera, for 4x5 Pictures, with 

Instantaneous Lens 75 00 

The price for the 3|^x4j Scovill Detective Camera, with Instan- 
taneous Lens, three double Dry-plate Holders, and room in 

the case for six double Holders 50 00 

The price for the 4x5 Scovill Detective Camera, with Instantaneous 
Lens, three double Drj^-plate Holders, and room in the case 

for six double Holders 60 00 

Many amateurs have declared that the pleasure of picture-taking was not 
fully revealed to them until they had procured and tried one of the 
SCOVILL DETECTIVE CAMERAS. 



Scovill's Outfit for Making Lantern Slides consists of 

1 doz. Thin Crystal Glass. 

2 " Black Mats. 

1 package Black Adhesive Paper. 

1 doz. 3^x4^ Gelatino-Albumen Dry Plates. 

1 package S. P. C. P3'ro and Potash Developer. 

2 4ix5i Solid Glass Pans. 
1 lb. Hyposulphite Soda. 

The above, packed in wooden case, price complete $3 50 

For enlarging, reducing, or copying Negatives to make Lantern Slides, 
we recommend the use of one of the Scovill Enlarging, Reducing and 
Copying Cameras. 



SCOVILL'S AMATEUR SPECIALTIES. 



29 



The ScoTill Enlarpi M\m aii Copyii Cameras. 




When ordering, please specify number and sizes of kits wanted. 



Size, 61^x81^, Price, $30.00 
" 8x10, " 35.00 

" 10x12, " 48.00 



Size, 11x14, 
" 14x17, 



Price, $60.00 
72.00 



Size, 17x20, ■ - $90.00. 
Special sizes and styles made to order. 

The form of construction of this new Camera is made apparent 
by the illustration here shown. The experienced copyist will not 
need any such simple directions for use as we append. 



I>IRBCTIO]>(S FOR USH. 

To copy a negative in the natural size, place it in the kit on the 
front of Camera and button it in. Attached to the center frame 
of the Camera is a division upon which, on the side toward the 
Camera front, a Lens is mounted. Suppose this to be a quarter- 
plate Portrait Lens, the focal length of which we will suppose to- 
be 4 inches — draw back the center frame and the Lens twice the 
focal length of the Lens (8 inches); slide the back frame with 
ground glass the same distance from the center frame. To enlarge 
with the same Lens to eight times the size of the original, the 
center of the Lens must be 4-J inches from the negative, and the 
ground glass be 36 inches from the center of the Lens. To reduce 
in the same proportion, reverse and have 36 inches from the center 
of the Lens to the negative, and from the center of Lens to ground 
glass 4^ inches. 



30 



SCOVILL'S AMATEUR SPECIALTIES. 



KBLBURN GUN CAMERA, 



For 4x5 Pictures. 

(patented.) 



Price, S37.00. 



Gnnstock Attachment only $5.00, 




A popular method of hunting lately introduced is in conformity with the 
laws of Mr. Bergh's Society for the Prevention of Cruelty to Animals. It 
never results in the death or even maiming of fish, flesh, or fowl, yet all three 
may be easily bagged. The weapon used is a late invention called the gun 
camera. It consists of a small camera mounted on a gunstock and pro- 
vided with sights and triggers. Its ammunition is chemicals instead of 
powder and lead. It is both breech and muzzle loading, is light and simple 
in construction, and is used like an ordinar}' shot-gun. When a bird rises, 
it must be brought to the shoulder, a dead aim taken at the feathered ob- 
ject, and the trigger pulled. There is a slight shock as of an explosion, the 
bird flies on to cover unharmed, leaving its picture on the sensitive plate in 
the camera. It is all done in a moment of time. The plate is removed, 
another inserted, and the hunter is ready for the next object. The amateur 
may go forth with two dozen dry plates as his stock of ammunition. If he 
fire with precision at any stationary or moving object, he may be absolutely 
sure of bringing it down. — New York Tribitne. 



SCOVILL'S AMATEUR SPECIALTIES. 



31 



THE PETITE CAMERA. 




This camera was made to suit the refined taste of one of Vassar's fair 
students. The design on the part of the manufacturers was to reduce the 
impedimenta for'^an outing to the minimum, providing a 3^x4^ camera (to 
make negatives of suitable size for lantern slides), with single swing, fold- 
ing bed with patent latch, vertical shifting front, and other desirable im- 
provements. So well has the design been carried out that many ladies will 
follow the example of Vassar's pupils, and learn the fascination of picture- 
taking with one of these finel3'-polished mahogany cameras. Gentlemen 
in search of a pocket camera need not seek further. The Petite Camera 
and an enlarging camera will by many be considered a satisfactory and 
complete equipment for such photographing as they desire to do. 

Petite Camera with one double dry-plate holder, zx^A patent Register- 
ing Slides $12 00 

Same Camera with Scovill's adjustable (feather weight) tripod and 

canvas bag, with shoulder strap 17 00 



^WATERBURY I.EIVSES. 

Provided with a Set of Stops. 




Notwithstanding what may be said or imagined to the contrary, it is a 
fact that many of the most exquisite photographs ever produced have been 
taken by the single achromatic lens, which is composed of a bi-convex lens 
made of crown glass, cemented by a transparent medium to a piano-con 
cave lens formed of flint. 



No. A, Single $3 50 

" A, Matched pair 7 00 



PRICE. 

No. 



B, Single. 

C, " 



$4 50 
. 8 00 



32 SCOVILL'S AMATEUR SPECIALTIES. 



OUTFITS FOR INSTANTANEOUS PHOTOGRAPHY. 



Instantaneous Outfit No. 401. 

CONSISTING OF 

202 Outfit $32 00 

Peerless Lens 12 50 

Instantaneous Drop (wood) for front of Lens 1 50 — $35 00 

Instantaneous Outfit No. 402. 

202 Outfit 22 00 

No. 1 Darlot Rapid Hemispherical Lens 15 00 

Instantaneous Drop (wood) for front of Lens 1 50 — 38 00 

Instantaneous Outfit No. 403. 

202 O utfit 22 00 

Morrison's Celebrated B Group Lens, with metal Drop.. 35 00 — 56 00 

Instantaneous Outfit No. 404. 

203 Outfit 30 00 

No. 2 Darlot Rapid Hemispherical Lens 25 00 

Instantaneous Drop (wood) for front of Lens 1 50 — 5S 00 

Instantaneous Outfit No. 405. 

203Outfit 30 00 

Morrison's Celebrated C Group Lens, with Instantaneous 

metal Drop 40 00 - 69 00 

The same Chemicals and Printing Requisites can be procured for the 
above as for common Outfits. 



Lenses for OStaliiii Instantaneois Pictires. 



B Morrison's Celebrated Group Lens, with metal Drop each, $35 00 

40 00 
50 00 
16 50 
26 50 
36 50 
9 50 
Lenses, matched for Stereoscope "Work, per pair, 17 00 



C 

CC " 

No. 1 Darlot Rapid Hemispherical Lens, with wood Drop. . 

" 2 

" 3 
Imitation Dallmeyer Lens. 



SCOVILL'S AMATEUR SPECIALTIES. 



33 




Morrison's 
Wide-Angle View Lenses. 

Patented May 21, 1873. 

These Lenses are absolutely rectilinear; they 
embrace an angle of fully 100 degrees, and are 









" 


'"r''^ 




'••^"6'"- ^^' 




' 


No. 


Diameter 
of Lens. 


Size of Plate. 


Equivalent 
Focus. 


Price, 
Each. 






00.. 

c. 

1.. 


. .f inch. 
••* " • 
••1 " • 


. 24 X 24 
. 3' X 3" 
.4x4 


inches. 


. If in 

. 2i ' 
. 8 ' 


ches. . 


. $25 00 
. . 25 00 
. . 25 00 


1 


These 3 sizes will 
fit into 1 flange. 


2.. 


..1 •' . 


.4x5 




31 ' 




. , 25 00 


^ 




3.. 
4.. 
5.. 




. 44x 71 
. 5" X 8 
■ 6ix 8i 




• 4i ' 
. 64 ' 




. . 25 00 
. . 25 00 
. . 25 00 


These 5 sizes will 
fit into 1 flange. 


6.. 




. 8 X 10 




. 8 ' 




. , 30 00 






7.. 

8 . 


..li " . 


.11 xl4 
.14 x 17 




• lOi ' 
.14 ' 




. . 40 00 
. . 60 00 


[ 


These 2 sizes wiU 
fit into 1 flange. 


9.. 
10.. 


..If " . 


.17 x20 
.20 x24 




.17 ' 
.22 ' 




.. 80 00 
..120 00 


\ 


These 2 sizes will 
fit into 1 flange. 



Remarks. — Nos. 1 to 6 are all made in matched pairs for 
stereoscopic work. The shorter focused Lenses are especially 
adapted for street and other views in confined situations. For 
general purposes, a pair of No. 5 Lenses will be found most 
useful. 



Morrison's Instantaneous Wide- Angle Yiew Lenses. 

With full opening, these lenses have all the extreme depth for 
which the Morrison Regular Wide-Angle Lenses are noted. They 
work with extreme rapidity, and will cover an angle of 90 degrees 
sharp. Furnished with a pneumatic drop and a set of 
diaphragms. 



Diameter of Lens. 



■J inch. 

1 '' 

H " 



Size of Plate, Full 
Opening. 



4x 
4x 
5x 8 
8x10 



4 inches. 

5 " 



Size of Plate when 
Stopped Down. 



5x7 inches. 

8 x 10 " 
10 x 12 " 
14 x 17 " 



Focus. 



5Ai 



^in. 



10 
13 



Price. 



130 00 
35 00 
40 00 
45 00 



With 
Rotary 
Exposer. 



$60 00 
65 00 



Protectors for any of above Lenses $12 00 

" C Group Lenses 12 00 

CC " " 17 00 



34 SCOVILL'S AMATEUR SPECIALTIES. 



Darlot Hemispherical Wide-Angle Rectilinear View Lenses. 

These Lenses embrace an angle of 90 degrees, and 
are valuable for taking views of buildings, interiors, 
etc., in confined situations, where those of longer 
focus cannot be used. 

Back Focus. Size View. Price. 

No. 1, 2^ inches For Stereoscopic Work, each . . . .$12 50 

" 2, 3 " " " " 15 00 

" 3, 5 " 8x10 20 00 

" 4. 8 " 10 X 12 25 00 




Darlot Rapid Hemispherical Yiew Lenses. 

These Lenses embrace an angle of from 60 to 75 degrees; are 
quick-acting, perfectly rectilinear, and provided with central stops. 
Will be found very fine lenses for landscape and outdoor groups; 
also for copying engravings, maps, architectural subjects, etc. 

Back Focus. Size View. Price. 

No. 1, 5^inches 5x 6 115 00 

" 2, 9 " 5x8 25 00 

"3,10^ " 8x10 35 00 

No. 1 can be had in matched pairs for Stereoscopic work. 



ScoTlU's "Peerless" Quiet Actiii Stereoscopic Leises, 

FOR PORTRAITURE OR VIEWS, 

The Lenses are especially designed for Stereoscopic Photography, and 
are so constructed that they will work well for interiors or exteriors. 

They are particularly adapted for instantaneous work. 

Diameter of Lenses, 1^^ inch ; focal length, 33^ inches. 

By removing the back lens and substituting the front combination, a 
focal length of 5)^ inches is obtained. 

They are supplied with six Waterhouse diaphragms in morocco case. 

Price, per pair $25 00 | Waterbury View Finder $3 00 

ALL STYLES OF LENSES SUPPLIED. 



A New Departure in Morrison Wide-Angle Lenses. 

{Extract from Photographic Times, Vol. jciv, page 277.) 
Opening the velvet-lined morocco case presented to us for our inspec- 
tion, we find partitioned-ofF space containing an ordinary 5-inch Morrison 
Wide-Angle Lens, on which the front and back combinations are distinctly 
marked with the figure 5. Beside this, in cells, are four mountings with 
lenses of varying focal lengths, each marked in white with a number. By 
unscrewing the back combination marked 5, and putting in its place the 
mounting marked 6, a lens of 6-inch back focus is obtained. Again, by 
removing both these cells and replacing them with the two marked 8. a 
lens of 8-inch back focus is the result. By screwing in the front combin- 
ation marked o and the back combination marked 4, a lens of 4-inch back 
focus is obtained. Putting a front combination marked 8 and a back 
marked 6, a focus of 7 inches is produced. Thus the operator has a choice 
of five focal lengths with the one lens. Price for the whole, $80. 



SCOVILL'S AMATEUR SPECIALTIES. 



35 




'76 Camera, with Morrison Lens and Water- 
tury View Finder. 



36 SCOVILL'S PUBLICATIONS. 



Photographic Publications. 



SCOVILL'S PHOTO. SERIES. 

Price, 
Per Copy. 

HOW TO MAKE PHOTOGRAPHS, containing full instructions for making 
Paper Negatives. (One hundred and twenty thousand.) Sent free to any 
practitioner of the art. 

No. I.— THE PHOTOGRAPHIC AMATEUR. By J. Traill Taylor. A Guide 

to the Young Photographer, either Professional or Amateur. (Second Ed.) f o 50 

No. 2.— THE ART AND PRACTICE OF SILVER PRINTING. (Second Edition) 50 

No. 3. — Out of print. 

No. 4.— HOW TO MAKE PICTURES.— Third edition. The ABC of Dry-Plate 
Photography. By Henry Clay Price. Illuminated Cover, 50 cts. ; 
Cloth Cover 75 

No. 5.— PHOTOGRAPHY WITH EMULSIONS. By Capt. W. De W. Abney, 
R.E., F.R.S. A treatise on the theory and practical working of Gelatine 
and Collodion Emulsion Processes i 00 

No. 6. — No. 17 has taken the place of this book. 

No. 7.— THE MODERN PRACTICE OF RETOUCHING —As practiced by M. 

Piquepe, and other celebrated experts. (Second Edition) 25 

No. 8.— THE SPANISH EDITION OF HOW TO MAKE PICTURES.-Ligeras 

Lecciones sobre Fotografia Dedicados a Los Aficionados i 00 

No. 9.— TWELVE ELEMENTARY LESSONS IN PHOTOGRAPHIC CHEM- 
ISTRY.- Presented in very concise and attractive shape 25 

No. 10. -THE BRITISH JOURNAL PHOTOGRAPHIC ALMANAC FOR 1883. 25 

No. II.— Out of print. 

No. 12.— HARDWICH'S CHEMISTRY.— A manual of photographic chemistry, 
theoretical and practical. Ninth Edition. Edited by J. Traill Taylor, 
$2.00; Cloth 2 50 

No. 13.— TWELVE ELEMENTARY LESSONS ON SILVER PRINTING. 
No. 2 has taken the place of this book. 

No. 14.— ABOUT PHOTOGRAPHY AND PHOTOGRAPHERS.— A series of in- 
teresting essays for the studio and study, to which is added European 
Rambles with a Camera. By H. Baden Pritchard, F.C.S 50 

No. 15.— THE CHEMICAL EFFECT OF THE SPECTRUM. By Dr. J. M. 

Eder 25 

No. 16.— PICTURE MAKING BY PHOTOGRAPHY. By H. P. Robinson. 
Author of Pictorial Effect in Photography. Written in popular form and 
finely illustrated. Illuminated Cover, 75 cts. ; Cloth i 00 

No. 17.— FIRST LESSONS IN AMATEUR PHOTOGRAPHY. By Pkof. Ran- 
dall Spaulding. a series of popular lectures, giving elementary instruc- 
tion in dry-plate photography, optics, etc 25 

No. 18.— THE STUDIO: AND WHAT TO DO IN IT. By H. P. Robinson. 
Author of Pictorial Effect in Photography, Picture Making by Photog- 
raphy, etc.; Illuminated Cover 75 

No. 19.— THE MAGIC LANTERN MANUAL. (Second edition.) By W. I. 

Chadwick. With one hundred and five practical illustrations ; cloth 75 

No. 20.— DRY PLATE MAKING FOR AMATEURS. By Geo. L. Sinclair, M.D., 50 

THE BRITISH JOURNAL ALMANAC, AND THE PHOTO. NEWS YEAR- 
BOOK OF PHOTOGRAPHY FOR 1886. For the two 75 

ART RECREATION —A guide to decorative art. Ladies' popular guide to home 

decorative work. Edited by Marion Kemble 2 00 

THE FERROTYPER'S GUIDE.— Cheap and complete. For the ferrotyper, this 

is the only standard work. Seventh thousand 75 

THE PHOTOGRAPHIC STUDIOS OF EUROPE.— By H. Baden Pritchard, 

F.C.S. Paper, 50 cts. : Cloth i 00 

PHOTOGRAPHIC MANIPULATION.— Second edition. Treating of the practice 

of the art and its various applications to nature. By Lake Price i 50 

HISTORY AND HAND-BOOK OF PHOTOGRAPHY.— Translated from the 

French of Gaston Tissandier, with seventy illustrations 2 50 



SCOVILL'S PUBLICATIONS. 37 



AMERICAN CARBON MANUAL.— For those who want to try the carbon print- 
ing process, this work gives the most detailed information 2 00 

MANUAL DE FOTOGRAFIA. By Augustus Le Plongeon. (Hand-Book for 
Spanish Photographers.) Reduced to $1.00 

SECRETS OF THE DARK CHAMBER. By D. D. T. Davie i 00 

HOW TO SIT FOR YOUR PICTURE. By Chip. Racy and sketchy 30 

THE PHOTOGRAPHER'S GUIDE. By John Towler, M.D. A text-book for 

the Operator and Amateur i 50 

A COMPLETE TREATISE ON SOLAR CRAYON PORTRAITS AND 
TRANSPARENT LIQUID WATER COLORS. By J. A. Barhydt. Practical 
ideas and directions given. Amateurs will learn ideas of color from this book 
that will be of value to them. And any one by carefully following the directions 
on Crayon, will be able to make a good Crayon Portrait 50 



WILSON'S PHOTOGRAPHIG PUBLICATIONS. 

WILSON'S PHOTOGRAPHICS— By Edward L.Wilson. The newest and most 
complete photographic lesson-book. Covers every department. 352 pages. 
Finely illustrated 4 00 

THE PROGRESS OF PHOTOGRAPHY SINCE THE YEAR 1879— By Dr. H. 
W. VoGEL, Professor and Teacher of Photography and Spectrum Analysis at the 
Imperial Technical High School in Berlin. Translated from the German by 
Ellerslie Wallace, Jr., M. D. Revised by Edward L. Wilson, Editor of the 
Philadelphia Photographer. A review of the more important discoveries in 
Photography and Photographic Chemistry within the last four years, with 
special consideration of Emulsion Photography and an additional chapter on 
Photography for Amateurs. Intended also as a supplement to the Third Edition 
of the Handbook of Photography. Embellished with a full-page electric-light 
portrait by Kurtz , and seventy-two wood cuts 3 00 

PHOTOGRAPHERS' POCKET REFERENCE BOOK. By Dr. H. W. Vogel. 
For the dark room. It meets a want filled by no other book. Full of formulas^ 
short, practical and plain i 50 

PICTORIAL EFFECT IN PHOTOGRAPHY.-By H. P. Robinson. Fortheart 

photographer. Cloth, $1.50; paper cover 100 

WILSON'S LANTERN JOURNEYS.— By Edw.ard L. Wilson. In two volumes. 
For the Lantern Exhibitor. Gives incidents and facts in entertaining style of 
about 2,000 places and things, including 200 of the Centennial Exhibition. Per vol. 2 00 

THE PHOTOGRAPHIC COLORISTS' GUIDE.— By John L. Gihon. The 

newest and best work on painting photographs ; Cloth i cq 

PHOTOGRAPHIC MOSAICS, 1886. Published annually. Better than any of its 

predecessors. Cloth bound, $1.00; Paper cover cq 



PHOTOGRAPHIC REFERENCE BOOKS. 

AMERICAN HAND-BOOK OF THE DAGUERREOTYPE. By S. D. Hum- 
phrey. (Fifth Edition.) This book contains the various processes employed in 

taking Heliographic impressions 10 

THE NEW PRACTICAL PHOTOGRAPHIC ALMANAC FOR 1^73. Edited 

by J. H. FiTZGIBBON 25 

AMERICAN PHOTOGRAPHIC ALMANAC FOR 1868 25 

MOSAICS FOR 1870, 1871, 1872, 1873, 1875, 1877, 1878, 1881, 1882, 1883, 1884. Per copy. 25 

BRITISH JOURNAL ALMANAC FOR 1878, 1882 " 25 

PHOTO. NEWS YEAR-BOOK OF PHOTOGRAPHY FOR 1871, 1882. " 25 

THE PHOTOGRAPHER'S FRIEND ALMANAC FOR 1873 25 

AMERICAN ALMANAC OF PHOTOGRAPHY FOR 1864. Edited by Charles 

Waldack .... oi; 



ISSUED EVERY FRIDAY. 
Ktonthly Edition Issued the Last Friday in the Month. 



TJe photogi^apMc Tiige^ 



i^nd pERlCi^W pHOTO^I(ApHEI(, 



^/IV? 



A SPEGIMEK GOPY FREE. 



.4}k- 



<^^ 




<^^ 



SXJBSCRIPXIONS. 

One Copy Weekly issue, postage included, to 

all points in U S. or Canada $3.00 

One Copy Monthly issue, postage included, to 

all points in U. S. or Canada 2.00 

Weekly issue to foreign addresses (postage in- 
cluded) 4.00 

Monthly issue to foreign addresses (postage 

included) 3.00 

Single Copy, Weekly 10 

" " Monthly 25 

ADVERTISING RATES. 

Size of advertising pages, 6JXQi- inches ; outside 

size, SJxiTf inches. 

Cards, 2j-.\3 inches, per insertion $2.50 

One page, each insertion, in either Weekly or 

Monthly edition 20.00 

Outside page, special terms, in either Weekly 

or Monthly edition. 
Busi7iess Notices, not displayed, per line 15 

^GoVill Iffg. Co, pul)Ii?5Bi«^, 

W. IRVINC ADAMS, Agent. 



^^ 



<$^ 



^ 



BOSTON PUBLIC LIBRARY 






3 9999 05493 481 3 



Boston Public Library 
Central Library, Copley Square 

Division of 
Reference and Research Services 



The Date Due Card in the pocket indi- 
cates the date on or before which this 
book should be returned to the Library. 

Please do not remove cards from this 
pocket. 



U- 



%U\^ ]m 



*^V-ir-; 



:4^„ > r 



'fy^. r^- 






"i .^ 








'^^^K 


^^^ 









*^':d' 






^- ■ -rJBL 



/,.• - > ^ 



:ii>^. V